I've found that Amazon's Kindle store has plenty to choose from, and iBook's will grow over time. The problem is that the ebook market is very much in it's infancy at the moment, but should improve as devices like the iPad and Kindle become more popular.
As usual though, much of this growth will hit the US first, not the UK. I'm hoping Apple can work out deals with the UK publishers and start getting the iBooks store properly populated.