ask.co.uk maybe. The real answer is google has about a 75% market share, Yahoo has 10%, MSN has 5% and the others have the rest. These figures are based on my observations of UK search traffic rather than the official figures
Yeah Ask comes 4th. I do get some from Wanadoo, NTL, Blueyonder and some other ISPs as well but most of them use Google's engine anyway.
ASK do TV ads so you get some from there and Wanadoo is expanding its subscriber base so that will become more popular, but as with most places the big 3 have the UK wrapped up.
I have a friend that moved over to the UK for a job for a few years and is now back. She said she used http://www.webfetch.com