According to a report from Hitwise, the largest are Google, MSN
Search, Yahoo! Search and Ask Jeeves (now Ask). Together they
represented 81% of all search traffic for the UK in September 2005.
Google?s UK site had 63% of this group of four.
In the UK, 63 per cent of visits to a search engine in the three
months to May  went to Google, compared with 47 per cent in the
You may also be interested in this report from Nielsen Ratings ranked
by click-throughs per searcher.
From my own experience I can say these appears accurate. I have a UK
based web site and referrals from google.co.uk outstrips by far any
other search engine.
I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.