I hear what you are saying. Firstly, I think that the guys importing cheap carbon frames and whacking a 100% mark-up on them are chasing a different customer. When someone goes down the custom build avenue they are looking for something special. Something unique, beautifully crafted and without compromise. Its like comparing a Saville Row suit with George at Asda. If you want off the peg you buy off the peg.
Secondly, how many UK frame builders have been able to mix craftsmanship and a love of the art of framebuilding, with sound business sense, good marketing and promotion? Some of the best framebuilders I know don't even know what a website is, never mind have one.
When all is said and done, there will always be a Market for something of exceptional quality and beauty. You might not end up a rich man but if you can make a modest living whilst putting smiles on faces then who needs fiscal wealth?