OK, don't panic! There will be an easy to use solution :)
Long post follows and I'll follow up with a summary of recommendations and some videos to look at, so you can choose depending on whether you want more info or less!
Halfords are better than nothing for fitting demos, but they do not sell 25kg rear facing seats in the first place, and they seem to have massively reduced their stock of 18kg rear facing seats now as well, which is a shame. If they stock the seat you want then absolutely go and have a look, because it's useful to see it in your car and isofix seats are actually fairly difficult to install wrong. Do you have a John Lewis or Smyths?
I don't believe there are any (rear facing) seats in the UK which are isofix up to 18kg and belted beyond that. There was one but it got withdrawn from sale last year.
The ones which go up to 7 years generally are harnessed until 18kg and then a booster seat after that. And not a full sized booster, just up to age 6/7, so you'd need to buy another eventually anyway. There are a lot of these from the cheapo companies, and generally they are not a great choice, unless you're on a tight budget or looking for a spare/travel seat.
With your son's last weight and height he's big for his age, between 75th and 97th centile or if you prefer he's in the top 10-20%. This sometimes slows down once they are walking, but assuming it doesn't, you can assume that an 18kg "up to 4 years" seat will last him until around three, maybe three and a half. If you're lucky, have a seat with a long shell and he times his growth spurts right you might get a bit longer. In a shorter seat or if he has a growth spurt at the wrong time, then potentially less, it's just an estimate. A seat "up to 7" will probably last him until around his sixth birthday, maybe a few months before, maybe a few months after.
Maxi Cosi is a good brand and their seats which forward and rear face generally get good safety tests, so I would trust the Mica. It is i-Size though, which limits you to exactly 105cm in height even if he has more space physically in the seat. Your son would be estimated to reach this somewhere between 3 and 3.5 years old.
As your son is tall I think Joie 360 Spin or Joie Spin Safe (the same seat, but rear facing only and with side impact protection wings) would be a good choice for you. These have a deep seat and tall back/shell, I've heard of children getting to 110/112cm in the Joie 360, which would roughly be an extra 6 months' usage for a tall child. On the other hand, the Joie seats sit very close to the seat back (not much leg room) so could feel cramped rear facing for a taller child. If you wanted more leg room in the 18kg bracket, the Axkid Modukid is worth a look, but that's i-size like the Maxi Cosi Mica, and not a rotating one.
Cosatto Den is only rear facing up to 83cm which at a guess would be too little for your LO already. And then it is forward facing only to 95cm in total, which is smaller than the average 3 year old! Your LO would probably be just over 2 when he outgrows this seat, so does not seem worth it for you. Cosatto also do not get very good safety results on their toddler seats.
No worries about questions! I like answering them :) I just got interested in car seats when DS1 was little (he's 11 now) because I didn't drive but I wanted a way to keep him rear facing, there wasn't a huge amount of choice on the market then. I'm interested in the phyiscs/engineering side but it also fascinates me the human factor - how car seats are designed vs how they are actually used. I'd like to get into the industry or do some study of it eventually. Lastly, the regulations are such a mish mash that it seems absolutely impossibly confusing for anyone coming to it now. It was not as complicated 10 years ago (IME) but with everything that's been built in and put on top of everything else there is now such a huge array of options no wonder people are confused. I've followed it from back then, and done some digging into what the law/changes were previously to that time as well so I feel like I have a good understanding of the whole situation.
So I indulge my interest now by reading car seat questions on parenting forums and reading car seat groups on Facebook, answering other people's questions gives me a chance to research and find out things I'd never have needed to know on my own or thought of looking up. Then I have all the accumulated knowledge from general reading. If I do ever get the chance to study/work in this area, I'm hoping that will be of benefit, plus I just find it fun and I like helping people find the info they need to make a decision.