Go and ask the butcher! But, you'll have to figure out what the beef are eating. I asked about getting lamb trimmings from them, Whole Foods even, and I got the fat for $1 a pound, so not bad at all. I went for the lamb since it was grass fed, but then you'll have a stronger flavored fat to deal with.

Eat some organ meats (heart, liver, kidney) for the vitamins, but the fat has got good stuff in it too.

You want to ask for suet, which you should absolutely be able to get in the UK. Render it into tallow (beef fat) the same way you would lard (from pork back fat or leaf lard)--that is, cut into small chunks and cooked over very low heat in a big pot. (Look up directions via Google for rendering lard. It's super easy.)

Yes, you should be able to get it. We use it when we make deer sausage.

Do a search for not local Butcher shops, not many of them around any more but for butcher/slaughter houses which get to keep some of the animal they butcher and package for thew local farmer to sell, as part of the fee, some sell from a store front.

Why not call around and ask the butchers in your area? Ask them if they can supply grass fed meat or fat. There are lots of fatty cuts of meat other than steak that are reasonably priced. The cuts may be different in the UK than in the US though, so I won't advise you on which; just ask the butcher which cuts are fattier. You're in Wales, so you should also be able to get great fatty cuts of lamb and mutton.

Added: Here are some grass fed beef and other animal producers in the UK. Two are in Wales. http://www.seedsofhealth.co.uk/resources/meat/index.shtml You can find more by googling grass fed beef wales

Added: This thread http://community.myprotein.com/diet-nutrition/29197-grass-fed-meat-sainsburys-tescos-other-supermarkets.html indicates that grass fed (and finished) beef may be the norm in the UK unlike the US.