Anything which sounds German or at least Continental. Avoid drinking any of the local pish, as after 6 or 7 pints of it you begin to sound like Dick van Dyke and develop a strange desire for 'buttons' and 'jellied eels'...cor blimey.
1. Avoid anything pretending to be German, Irish or Australian
2. Avoid anything chilled / cold
3. Avoid anything artificially frothy.
You can't go that far wrong if you follow those rules - plenty of good suggestions made above - and it depends whereabouts in the UK you are, as Beers tend to be very local; many simply don't travel further than a few miles at all well. Bottled beers? Again, lots of nice ones easily available in supermarkets - anything pretty much by Shepherd Neame or Badger, Fullers or Youngs, for a start.
Oh, and I forgot No. 4. Most beer made north of Watford is pish.