Looking for an amazing SE boarding school....

OP you’ve had loads of suggestions and I suspect there’s a risk you’ll get very confused and you’ll end up end,essay reading websites and attending open days. I looked at the websites of some of the ones mentioned basically they all say pretty much the same stuff.
If I was you I think I’d start of by deciding if I wanted a proper full boarding school, if yes that really limits your options very significantly or whether weekly boarding will work for you if it’s in the right location. If you want the former then you might have to widen the distance you’re happy to travel as ultimately of those suggested above and within your proposed area only Marlborough or Radley (which has not been suggested on here) meets the criteria of proper full boarding. This could present you with a problem as 1. neither might not be your cup of tea and 2. your head has described Marlborough it as “aspirational”.
If you can entertain weekly boarding and Brighton College which you mentioned at the beginning is weekly boarding then you’ve got plenty of choices you just need to find a school that suits you and your DS. I think I would start of by drawing up a list of must haves, and look at 4 1 aspirational, one achievable with consistent work from your DS, one your virtually guaranteed a place at and I would always suggest 1 you don’t like the look of I learnt the most from those even if it’s “this is what I definitely don’t want”. Go and visit a few times IMO not on open days if possible ask lots of difficult questions, if it matters to you ask, don’t be wooed by jaw dropping facilities and Medieval manuscripts, talk to pupils not the polished and buffed high achieving ones wheeled out to show off the school, watch staff with pupils how do they interact with each other, do you like the atmosphere of the place, the staff you meet, do you feel comfortable and can you see what the school is really about, will it work for your DS?
It’s defintely worth listening to what others say and think about a school but ultimately do not be totally swayed by them unless your saying you want full boarding and a current parent is saying it’s weekly boarding only or you must have a basketball club and a current parents is saying the school doesn’t have one. I believe that ultimately the senior school we choose will work best for us and our DCs when we choose the one that we genuinely feel is the best individual fit not the one that 1/2 the parents on the side of your prep school rugby pitch or on MN are raving about.

Also Seaford near Petworth and Lancing.

Bedales seems the obvious one is there a reason you've not mentioned it?

Christs Hospital hasn't been mentioned. South of Horsham.

Ruggles I don't have any personal experience of Ardingly but have heard positive things and it seems to fit what you want for most aspects (apart from not being in a town) so at least worth a visit.

Surely Oundle is too far for the OP?

Friend's daughter is at Worth and loving it.

A number of dd's prep year went to Worth and seem happy with it. It should be coed throughout by now. Not cheap though. Likewise Cranleigh which I think you mentioned earlier. Box Hill might be worth considering if your dc are outdoorsy rather than traditionally sporty.

I'd at least look at Sherbourne, despite the distance. I hear good things about it under the new headmaster and from friends' experiences it suits "jack of all" kids quite well. Admittedly it might be an issue if you want 2 kids at same school , one boarding one day, but wouldnt it just be easier to send them to different schools?

Lord Wandsworth College??? Suited our ds well.

Kings Canterbury or Oundle could also be considered. Neither ‘empty out at weekends’ and both are excellent for all rounders.

Dancing I've PMd you!
Kids getting drunk in boarding schools is pretty common, boarding schools cannot supervise children 24/7, many have lots of money in their bank accounts and will either purchase alcohol for themselves, or bribe older pupils or even the homeless to buy them alcohol. Some go home and drink it or steal it and bring it into school and I've even seen parents buy their kids alcohol and let them bring it into school.
Nearly every boarding school (Marlborough included) will have a drunk pupil in their medical centre most weekends. IME most of us as teenagers have got hopelessly drunk at least once, usually having drunk something vile and super sweet (mine was Baileys 30 + years ago, I've never drunk it ever again), you vomit really badly, wake in the morning feeling like shit, get told of by your parents or in this case school and your parents and if you've got any sense don't keep repeating the same daft mistake. I believe this is how we learn.

Presumably you've looked at Bedales?

It's a much more rounded school than its reputation suggests. I know two (v academic) teenagers who have done very well there. Whilst I wouldn't send a super sporty boy there, the sports provision is good for anyone who enjoys sport, and it's excellent for music/drama/art and outdoor work. I would take a look if you haven't already.

Ok thanks for the explanation Happy.

For those who are interested, Marlborough certainly do drugs test but only where they have reasonable grounds for suspicion. I don't know of any parent who has been fined for a drunk DC. The school threaten to charge parents if a DC needs to be cared for in the San because of drinking to excess but I have never heard of anyone needing to be admitted. Minor excesses result in being grounded. They do come down hard on any drinking in rooms or consumption of spirits. The students know the rules however so generally comply with them.

Wellington. Amazingly rounded, educate the whole child ethos.

Presume you’ve looked at Seaford? There’s been quite a change there in the last 3 years, now oversubscribed.... but it might suit you if you want one boarding and one day, as the coach system is pretty good....

Is Ardingly College 'a Fulham type' school ?
Does n't that do weekly boarding?

If people are going to be wary of Marlborough - I would also suggest being wary of Cranleigh, I have heard stories which would make me wary especially of a boarder. But I'm not sure how much the same could be said of any boarding school (but I hope not).

Was also going to suggest Bede,s . Family member doing very well there and apparently very good pastoral care.

Compared to Win Coll or St Edwards and contrary to what many say in here Marlborough isn’t liberal, there’s nothing wrong with this unless you’re specifically looking for a very liberal school.
There’s often stuff on here about drugs etc being a concern at Marlborough but this isn’t true as I know that they regularly randomly screen their pupils for drugs and come down hard on any found to be positive. Any pupils getting drunk, which is not uncommon at boarding schools, then parents get fined, least they used to.

I would just like to understand what people mean by the comments that Marlborough is not liberal. I have seen this mentioned a couple of times on threads and think this must date from some historical experiences.

We were very concerned about comments on Mumsnet about Marlborough. So much so that we very nearly didn't visit the school when looking for a 6th form place for my DD. She was however insistent that we went to look. We found a very different school to the one often portrayed on here and my DD has been incredibly happy there.

What about Bedes? They have a lot going on (have a look at the support for space science - and maybe also the animal management centre?) and are good at supporting a wide range of abilities. Might not be towny enough for you.