Perhaps this is a naive question, but don't most UK-based banks & brokers offer access to the U.S. markets? What advantages does opening an account with a U.S. firm offer to a UK resident (besides maybe geographic diversification of one's assets, which I certainly can understand)?
Regarding OptionsXpress, I've never been very impressed with their commissions on options. Perhaps their rates on straight stock are better; I haven't looked at that.
Would an account in Canada be easier for you to snare? ThinkOrSwim, a Chicago-based firm, has a subsidiary set up to serve Canadians. Their rates are far better than OX, and the customer service is excellent.