The Fuller's website has this to say about their London Porter:
Fuller's London Porter is ranked the number one tasting Porter in the world on ratebeer.com. Indeed, London Porter is a superb, award-winning beer having captured the gold and silver medals at the 1999, 2000 and 2002 International Beer & Cider Competitions.
The origins of Porter date back to London in the early nineteenth century when it was popular to mix two or three beers -- usually an old, well-vatted or "stale" brown ale -- with a new brown ale and a pale ale. It was time-consuming for the pub owner to pull from three casks for one pint, so brewers in London tested and produced a new beer, known as entire, to match the tastes of such mixtures. Using high roasted malts, entire was dark, cloudy and hoppy. It was also easily produced in bulk and ideally suited to the soft well water of London. Very quickly, it became popular among the porters working in Billingsgate and Smithfield markets. Gradually, the beer took on the name Porter in recognition of its greatest devotees.I popped the chilled can and poured the rich, black contents into a couple of glasses. Being a fan of Irish stout, I wondered how bad this one could be? It didn't smell bad, and the white, creamy head looked quite appealling. We both raised our glasses and took a hefty pull.
"Tarry!" came the exclamation from Brian as this one hit his tastebuds. Unlike Guinness which has a pleasant malt taste, London Porter is extremely malty and seemed to attack the tongue with a burnt chocolate taste as you drank it. While I was able to stomach mine, Brian had huge problems with this one. "I can't drink this," he stated. "It is like a Reese's commercial gone horribly wrong!" The heavy flavour of this one may be enjoyed by those who like their beer dark and malty, but keep in mind this is definitely no Guinness. As Brian dumped the contents of his glass, his final comment was "I don't need a tongue glove."