You are quite correct in your assumption that most of the heating and hot water services in the UK domestic systems are from a single source, either a gas, oil fired or solid fuel boiler either heating the water using an indirect storage cylinder with integral heating coil and separately controlled heating circuits or the increasingly popular combi gas or oil fired boilers. Separate heating and domestic hot water systems are more common in commercial or industrial premises however. Our regulations regarding work particularly in connection with work in connection with gas associated work are not just discouraged but are actually illegal carrying heavy fines and even imprisonment if carried out by un qualified people particularly those carrying out work for financial gain, but we have a very active DIY population in regards to Plumbing, certain aspects of electrical work and many other activities and being British, our natural tendency is flout some of these many regulations where we think we can get away with it.
I am a follower of Yahoo Answers and many of the UK questions are answered by Americans and from these, I assume that steam systems are still used in both domestic and commercial premises. I have never seen a domestic steam installation in all my 57 years in the industry and we were removing steam systems to be replaced by hot water heating systems in commercial premises back in the sixties, so steam is only used in process work in industrial premises in general now. Since the boom in warm air systems in the sixties and seventies these are rarely installed in houses these days due I think to low tec and cowboy installations. Baseboard heating also suffered due to the low quality of equipment.
We were installing underfloor heating in Cathedrals, offices etc back in the 1950,s but due to the advances in plastics, these systems are common in domestic premises now and sit heat pump and condensing boiler systems. I have kept my answer brief and have only scratched the surface.
Hope his is useful.
Regards. Mr. Microbore