Ok, more substantial reply.
Firstly, the best starting point is the British Goat Society who have an excellent links page to all the breed societies as well as various breeders and special interest groups.
In the UK goats are kept for a number of reasons. There are the dairy products - milk, cheese, yogurt, butter, fudge, etc. There is meat (often marketed as chevron.) There is fibre: mohair from Angoras goats, cashmere from others. Then there is the pet goat market. As with cattle, sheep, etc. various breeds have specialised in particular niches, although there are some multi-purpose breeds too.
Much of the goat produce you can buy in the UK is produced here. Certainly all of the fresh goat milk, butter and yogurt that places like Tesco are now stocking, as well as the meat where you can get it. Of cheese, much is imported from France, who are the real specialists, but a great deal is now produced here too, often to French regional recipies, but increasingly to locally devised recipies. All goat dairy products return a premium. Goat meat is harder to come by in the supermarkets, but the increasing interest from the ethnic markets is fuelling a demand that is difficult to meet and therefore produces pretty decent returns; the Muslim festival of Eid in November is eagerly anticipated by goat farmers!
I specialise in fibre goats: Angoras. This developed from a longstanding interest in fibre products: we had native breed sheep as well at one time and also have some camelids. In fact Mrs Townie has used animal fibres as diverse as sheep, goat, cat, llama, alpaca, rabbit and shetland pony. Mohair, the fibre of the Angora goat, is an outstanding fibre exceeding any man-made fibre in softness, resilience and flexibility. We take two clips per year each of 4-6 kilos for an adult and 2-3 kilos of the super-fine kid mohair. Return on kid goat fibre at current prices and assuming you add no value beyond bagging and selling on slightly exceeds the total cost of shearing and raising the kid. If you then sell on the meat, you are into a reasonable return. Kid mohair is produced over the first two clips (at 6 and 12 months, roughly) at which point the animal can be kept into adulthood or slaughtered at around 14 months returning a carcass plus a fantastic skin. If you are prepared to put some more effort in a more substantial return can be had from the fibre selling on yarn or finished products as many breeders do. The fibre is enhanced by blending with other fibres such as merino or southdown wool, alpaca or silk to give a range of interesting combinations of properties.
As I mention, Angoras have a secondary market in meat, although the premier meat goat is the Boer. In fact the Boer is made up from native African breeds such as the Nubian combined with Angoras. The meat of both is a very lean tasty meat similar to the best lamb and superior to the meat of dairy goats; many dairy breeders are now using an Angora or Boer buck unless they are breeding replacement milkers. The chief benefit of the Boer is the quicker growth to a larger carcass (obviously the Angora puts a lot of nutrition into the fleece.)
Downsides? Well it depends upon the breed. With Angoras, we note that they can have more foot problems than other breeds, not so much rot as a tendency to horn seperation and injuries. I believe this is more due to too soft a pasture and following the departure of our builders this year I built a 'mountain' of rubble for the goats to climb and this has actually helped to reduce the foot problems significantly - our llamas also love this mountain! As the fleece is clipped twice a year, in early Spring and early Autumn, there are inevitably periods when too little fleece means they are too cold or too much means they are too hot. Therefore adequate shelter to keep warm/cool is required. They don't much like rain although I have found giving them free access to the outside, unlike many breeders, they will venture out far more often than advertised! Ideally they should be indoors for kidding as young goats are more delicate than their sheepy cousins. Many dairy flocks are kept indoors all the time with outside access limited to hard standing; I don't personally believe this is ideal for such a curious and browsy creature, but the flocks in question seem to be doing quite well financially so I can't really argue with that!
Being browsers, rough grazing is the best thing for goats: a clean well fertilised sheep or dairy cattle sward IMO will not produce decent returns on any goat venture. For supplement, there are specialised feeds on the market, or a combination of straw and hay: never feed goats silage, it is far too rich and can kill them.
Feel free to ask any questions you may have. I'm pretty sure I've noticed a couple of other goatkeepers posting on here from time to time. You could also check out the various smallholding forums out there such as ACL or Smallholder magazine or River Cottage