In reply to the OP.
First off, welcome to the forum.
You have to keep your eyes open for hand made frames, as those 'in the know' tend to snap them up.
Roberts and Yates you know about, also from the UK you could look for Overbury's (as per Maksters post), Dave LLoyd, Rourke, Lee Cooper, Orbit / Sonic, Zinn, Bromwich, Kevin Winter, Fuquay. There are more, but those are the ones I can think of straight off.
Some of the early Orange bikes were fillet brazed, but they are rare and usually fetch a sensible price.
Probably the easiest to find would be a Yates Diabolo; these were a semi production frame, much like the Roberts Genesis , but both are nice looking, usually pretty well priced and from personal experience I can say that the Diabolo builds into a good fun bike.
You probably need to consider what age of frame you want, as bikes from the late 80's are very different to bikes only a few years newer, and by 2000 things were very different again.
Alternatively, you buy a frame you like the look of, and build it up however you want, that's what I do.
Do a search for British Steel in the riders bikes section, you'll see one or two nice frames in there.
"Try not to punch any clowns"