I dont have any experience procuring anything in the U.K., but here in the U.S. some grocery stores actually sell soup bones. Usually beef bones with some meat still on them...which is good.
You didn't indicate what kind of bone broth, but chicken bone broth is very good for you and cooks faster. Chicken skin is also very good for you so the pieces and parts of chicken like wings tips, neck and even the feet are good to toss in the pot for a rich broth. Roasting those parts first adds more flavor.
Obvioulsy buying bone-in meats and reserving the bones makes sense, but it can take a long time to save up enough bones, especially from ungulate mammals. Chicken is a little easier to accumulate the parts for a broth. I like to buy whole chickens and cut them up myself, nothing aggravates me more than buying a whole chicken and they didn't leave me the neck, liver, gizzards, etc., in the body cavity...I want those parts!
I'll either roast the whole chicken, pick it clean of meat and use whats left for a broth, or cut the breasts of the bone, de-bone the thighs (the wings are my special treat but those bones go in the pot too after I've deep fried and eaten them) and usually will submit the legs to a broth because I dont have much use for them otherwise, except picking the meat for chicken salad after roasting. When I cut apart a raw chicken, I roast the carcass and parts before boiling for broth. I also reserve any chicken skin I removed and slow roast that to release the fat and reserve it. Chicken skin fat is good for you fat and makes a great fat for cooking...eggs are very good cooked in chicken fat for instance.
I recommend looking in ethnic markets for stock bones. That's where I see them the most, especially in Asian markets, and they often also sell chicken feet, an excellent add to a broth (at least a couple) even when making beef or pork bone broth. The collagen and other stuff in them is very good for you and adds that gelatinous effect so many people are looking for in a bone broth.
