a well-backed UK exchange that could meet the regulatory burden would probably be successful. But without one, there are not enough UK users to justify setting one up.
Thankfully, you're wrong on the second part.
There are a huge number of UK Bitcoin users, we just buy and sell using MtGox, Virwox, BTC-E and various others. We'd prefer not to have to pay the extortionate transfer fees, and when Intersango still had free deposits via bank transfer, the majority of their deposits came from UK accounts for that reason.
The market is wide open for a decent UK based exchange, and I wish all the luck in the world to SBC.