Hooray for British Shorthairs! I've got two of them, a brother and sister pair, since they were 5 months old. Like yours, they were neutered at 6 months old, and they're 3.5 years old now. On recommendation of the breeder, we always fed them BARF-food - fresh meat frozen into a sausage-like shape, which we had to thaw before feeding it to them (BARF stands for either 'Bones and Raw Food' or 'Biologically Appropriate Raw Food' - the idea is that cats naturally eat fresh meat plus a few bones, and that this diet provides all they really need). Additionally, they always had a supply of dry food. We never gave them kitten food, nor did their breeder, since BARF is very close to what a cat would eat naturally. So I guess it really depends more on what type of food you're feeding than whether you're feeding kitten or adult food.
Unfortunately, though, about 1.5 years ago our tomcat developed urinary tract problems a short while after we had switched to a new brand of dry food (and I'm pretty sure it was the dry food). Ever since then, he's on a vet-prescribed special diet, and he gained loads of weight (he's 7 kg right now, even though he only gets tiny portions). We kept his sister on BARF, but gave her more wet food and less dry food (of another brand), and she is doing absolutely fine, she's not fat at all. My point is that the food prescribed by the vet is chock full of grains, and while it guarantees that he won't have any urinary problems anymore, it doesn't seem to be very healthy on a very different level (I'm just too scared to take him off the diet food and switch him back to BARF because I'm afraid I might notice too late).
So whichever food you choose for your cat, make sure it's as natural as possible, contains very little or preferably no grains at all, nor any sugar, and lots of water. That should go a long way in providing him with all the nutrients he needs, while it will at the same time prevent him from consuming calories that he can't process (grains and sugars) and that will make him fat.
This page gives a lot of good information on feeding cats: http://www.catinfo.org/