Google Affiliate Network (one of the alternatives to AdSense LINK, which lets you deal with specific, relevant, advertisers) has signed up 30+ UK based advertisers. Ref: http://googleaffiliatenetwork-blog.blogspot.ie/2012/06/announcing-new-uk-advertisers.html
(When you're looking for them, don't forget to set Relationship status to "available", instead of the default "approved".)
Fingers crossed some of these will also be suitable for my Ireland-focussed blog!