Kinda requires three sets of answers for me.
My absolute favourites that you can't find easily in the UK, let alone in bars (aside from De Hems in London)
Kwak (Belgian)
Delirium Tremens (ditto)
Westvleteren 12 (again, Belgian)
In terms of what you can actually get over here:
London Pride, Tanglefoot, Abbot Ale
In terms of what you can actually buy in ****ty pubs/bars that don't have real ale or clubs:
Guinness, Stella, Grolsch I guess.