Cask Old Brewery Bitter is a bit of a rarity in London, if I recall...DB.
Depends on the area. In central London everything in Soho is keg I believe (the Red Lion used to be real but apparently now keg as well, according to beerintheevening.) but east of Charing Cross Road all have Cask OBB except the Crown on New Oxford St.
Fitzrovia/Marylebone - in other words north of Oxford street it is a mix.
From memory and some not visited for a long time, so not guaranteed it's as follows.
Cask
Dover Castle
Yorkshire Grey
Horse & Groom
Bricklayers
Blue Post
Cock
Keg
Champion
Angel in the Fields
Fitzroy Tavern
I've probably forgotten one or two as well