I've just bought and installed an Exposure Flash (and the Flare rear too). About to use it to commute in for nights for the first time in a few minutes - will let you know what I think of it, if you're looking at the 30-40 pound budget for a 'be seen' light.
I've been using the Blackburn Flea 2.0 up until now, but have been let down by it failing to charge properly and needing replacing twice (the seller says its a common problem, however have had no issues getting a replacement twice).