Welcome aboard, 'Rez.' Interesting query. We've dealt with mailers in the U.S. as well as UK, most of whom agree with the sentiment that the UK market 'lags quite behind' the U.S. as far as regulations, deliverability, etc - meaning for affiliates/mailers, there is more opportunity. . .but as you say, the population is significantly lower so the 'ceiling' is not as high.
Vis-a-vis performance, we've seen some amazing eCPMs from mailers, but our experience shows that backend lead buyers are not as consistent. Also, an important thing to remember is that the 'business cycle' is much slower there, as UK-based networks, agencies, etc are not as 'agro' as they are here, meaning you may need to wait a week for a response, a week to set up account, a week to get campaigns, live. . .be patient, but seek out those opportunities as lawd knows it's saturated as all get out here in the U.S.
Also I do know that some networks have really good UK campaigns, Affiliate.com, AdMarketers, Intela, TerraMatrixMedia. Reach out, or if you need any contacts hit me with a PM.
Best of luck, player.
Ev