Go to
http://www.beersofeurope.co.uk and order online Desperados beer, delivery anywhere in UK, or also at :
http://www.thefoodcompany.co.uk/homepage.htm
Also see the database at:
http://www.bottledbeer.co.uk and there's a ton of info about where to buy foreign beers.
Desperado also has an online direct shop - this is not from the brewers though - see: www.desperadosshop.co.uk
There are a few others selling it around the UK - try a good off licence as well - independents sometimes have it, not the 'main' high street ones.