You said on another thread that you plan on visiting the Black Friars pub so you will be pleased to know that the Black Frairs pub do serve Young's. I stopped off on the way home tonight and as well as Younds, the other regular beers are Old Speckled Hen, London Pride and Timothy Taylor's Landlord.
On a similar note, have you ever been to the Market Porter, nect to Borough Market? Whilst they don't serve Youngs, their regular beer is Harvey's of Sussex. However, they do have 4 or 5 guest beers at any one time and it is a great pub to try different UK beers that you wouldn't normally find in a London pub.