Commercial Beers

Currently, these are the traditional mixed culture beers available on the market:

A Le Coq Imperial Extra Double Stout
Greene King Olde (Strong) Suffolk English Ale

Although these are discontinued, you can still find some bottles around if you're lucky:

Courage Russian Imperial Stout (currently being brewed without wild yeast)
Gale's Prize Old Ale
Gale's Conquest Ale