Andrew-
Since you’re talking about advertising using pounds rather than dollars, I would suggest having a look at eSpotting. I’ve seen very good conversions with them.
7search and Goclick have produced a positive ROI for me, although 7search is becoming extraordinarily competitive considering the low volume of traffic they’re able to send.
There are some small gambling oriented PPC’s called bettorsearch and searchgambling. Avoid them.
I’d avoid Xuppa for sure. I’ve seen some return on ePilot and Kanoodle, but I don’t recommend them.
I think your best bet is to focus on one of the better ones until you start making money. The key is to have a large list of keywords and really well-written ad-copy.
Test test test and track and measure everything.