Ah, petitdonkey, when I suggested you name the school, I wasn't expecting it to be one I myself would know anything to comment on.. One of my DC is friends with a really good all-rounder sports boy in your son's year in Ashford. If indeed you are Ts mum then I will stay disguised and just say hi.
I think the key things you need to think about are what your child would gain whilst he is there, what impact being a day pupil will have on all of you, what impact being a boarder would have on him, and what impact it will have on his future life. I'd consider drawing up pros and cons under these headings.
Day pupils seem to stay busy at kings till 8.30/9.00, sometimes later. AFAiK there is still a sizeable waiting list of day pupils wanting to switch to boarding because going home is a pain (especially if it means a long schlep down the A28). I personally know mums who didn't really think these finishing times were true and who were totally shell-shocked when it happened to them. So then the fact that this is one of the few schools remaining with proper full
Boarding becomes quite key to the decision-making process. I think there is a chink of light in terms if this (floating exeats being tried this year) but any change here might come very slowly.
In your position, if you have one of those boys who is really up for everything, who you know will dive into two or three sports and pop up in the As or Bs, who will stick their hand up for drama, will join a choir or rock ensemble, volunteer for the debating society and so on, then switch to Kings. There will be value for money for kids like this. Likewise, an outgoing and sociable child will adapt well to the full boarding experience and will have the time of their lives. But otherwise, and particularly if you are thinking of a day option and you live near Ashford, the benefits are less clear-cut. Especially as the fees are on a par with Eton/Tonbridge, but academically the school looks as if it isn't quite in that league at the moment (although they have jumped up and down the top schools lists over recent years)
At the end of the day, your kids will probably pop out of ashford with the same a levels, heading for the same schools. They won't be part of the toffs crowd at Uni, for sure, but that doesn't strike me as a bad thing. And you will save an awful lot of money.
There is a whole other story about whether having a big school name behind you helps or hinders future life/outlook/Job prospects/social standing, but its so subjective that you and your DH will have to work through that one by yourselves to agree a view of what you think the impact is likely to be and what value/detriment you assign to it.
Feel free to PM if you want any detailed info. If you do decide on kings, you will do a lot worse than Galpin's, or Carlyon as a house choice (Carlyon not very pretty, mind. It's all about the housemaster and their families with these two recommendations).