I don't think any of this is a US/UK thing. I know plenty of people in the US who live at home into their 30s and always consider their parents' house home.
Pooky_girl, you and I are in similar situations. I'm 27 and he's 24, his earliest memories are of his parents arguing. He has also seen examples of really bad relationships in his family and doesn't trust any marriage to be good. To him Marriage=miserable + divorce. He always thought that he wanted to be married and have children, but last spring we started looking at rings. We were planning to get married this summer. However, his mom and step-father came over to visit and he talked to them about it and their reactions were horrible. They told him "It is F*$#ing ridiculous to get married at your age." Nice, huh? It ended up being exactly what he needed to bring all of his fears about marriage to the surface and I have yet to see my ring and I don't know when marriage will happen. My mom has terminal cancer, which makes it a lot worse. It's taken a lot of discussion and arguing, but he's finally realizing he needs help and is going to get it. One thing that really helped us is this book called Lies at the Altar by Dr. Robin Smith. It has almost 300 questions that this marriage counselor things every couple should discuss before marriage. some of them seem like common sense (and we had actually discussed all but about 2 or three of them before), but apparently not everyone discusses them. I found it best to make him answer first. Going through each of those questions, from a counselor who has helped thousands of couples, really helped show him that we were in a great place, better prepared than a lot of people, and the issue is with his fears. Granted, I think the situation with my mother makes it a lot more of a high pressure situation than it would be otherwise, but the fears would be there anyway. Good luck!