There are thousands of ales in the UK. My advice would be to find the best pubs/bars in the local area, because cask ale dominates the UK scene and it's a volatile product that needs careful handling and often doesn't get it.
You can list the top rated beers by country and region on ratebeer.com, I find this useful when travelling. It's a reliable source, I find. Beers by Harbour Brewery and St Austell are highly rated in Cornwall. Fullers, the Kernel, Beavertown, Partizan, Weird Beard are the top rated ale brewers in London. In the Cotswolds you may well find beers by Arbor of Bristol, and the Bristol Beer Factory.
You can also find Places to drink on ratebeer.com but this is less reliable as there are very few reviews of places. Try the Camra website, or buy the Camra 2014 guide. Hopefully people on here can advise the best pubs in St Ives, the Cotswolds and London. If you nip up north to Manchester I will be able to tell you what and where to drink. :wink:
Perfectpint.co.uk is a good site for finding pubs and ales.
You could also use the Camra Good Beer Guide Mobile app or the Caskfinder app. The Camra app does the best job of locating the best pubs near me.