The best place to start would be to head to your nearest library & ask for the Yearbook of Tourism Statistics and the Compendium of Tourism Statistics. Both are produced by the World Tourism Organisation & are the most accurate & up-to-date you will find (which means up to 2002).
If you happen to be in or near London, Tourism Concern have a fantastic library with loads of material, much of which you simply won't find elsewhere in the UK. Even if you can't visit them they offer information searches for students (for a small fee).
http://www.tourismconcern.org.uk/resources/resources_informa tion_request.htm
You could also check out the WTO's website
http://www.world-tourism.org but you won't get much for free
And you might find some useful stuff at Visit Britain (the new name for the British Tourist Authority)
http://www.bta.org.uk/