Dragon BBQ at the Broadway Plaza, Birmingham was really good. Massive selection of dishes and they even had about ten types of sauces. Dessert choice was good - cream cakes, jelly, icecream, fritters. We used t go in the daytime and it was a fiver I think. If you go in the evening they also have a bbq, which you can supplement with the buffet as well, I didn't ever go in the evening, and it was dearer but I bet it is worth it still.
I used to go loads of times to two in Greenwich. One is called Wing Wah, near the old Greenwich hospital, and the other is Saigon Buffet, in the town centre. Both very good, a fiver in the day, and I think it was £8/9 in the evening. Daytime was good enough, and the evening had even more dishes, like salmon, muscles, and such like.
I went to one in Soho, I can't remember the name of it but it was awful. They only had about 10 dishes, most of which were padded out with LOADS of onions. The food was minging. It was only a fiver but I think they should paid ME a fiver to eat that slop!
I don't go to buffets anymore as I do prefer freshly cooked food.