Porter is currently an unfashionable beer style, as far as the mainstream goes, so in the larger 'chain' pubs you'd be unlikely to find one.
However, that's not to say there's none to be found! Your first step is to find the right kind of pub, for which there are many resources available; the best physical one I would say would be CAMRA's Good Beer Guide (bookshops everywhere). Online your search terms would be 'real ale', 'craft beer', and of course 'london pub'.
Here are some specific resources which should help:
Beer in the Evening - general pub reviews
Fancy a pint - pub reviews with an ale-drinking preference
The GBG pubs of London - but without the commentary that the book itself provides.
Finally, you could always cheat and look in a supermarket for a bottled version - not as good as the real thing, but consistently a decent pint.
Finally finally, if you're in East London I would personally urge you to try something by the East London Brewing Company - they don't currently do a straight porter, but their mild is worth a special trip, to my mind.