A site where a man with far too much interest in beer gets to write about it.
Beer or ale? Ale I'd take a guess at: Various Badger's. Hobgoblin. Pedigree. Something from Adnams. Fullers London Pride. Greene King IPA. I'd love to here Hook Norton Bitter or Old Hooky. Mann's Brown and Newcastle Brown based on combined on-/off-trade sales?
Best selling... if we are talking purely beer and not lager then ill go for the list including black sheep, old perculiar, speckled hen, hoegarden, hobgoblin and bass as they are all avaible in supermarkets.
In sales volume or sales turnover?The big boxes of bottled lout will sell more volume, but at 30-40p a bottle. The premium pong will sell less at £1.50-£2.00 a bottle, but with higher margins.
If we're just talking bottled ales, I'd say Newcastle Brown, Old Speckled Hen, Hobgoblin, Pedigree, Tanglefoot and Bombardier would probably all feature on the list.