I found www.undercovertourist.com to be very good. Although their prices are in dollars, unlike some other US sites they quote an amount in pounds and guarantee you will not pay any more than the price quoted (regardless of exchange rate moves).
To be honest there is not a lot in it, youcan buy them cheaper beofre you go, but you need to look for speical offers depending on how long you want to go for.
Just wanted to add that my tickets arrived this morning (I ordered them on Friday). Excellent service and the amount on my statement was actually a couple of quid LESS than was estimated.
hi
i'm trying to find a site that sells the universal 2 day ticket (one which doesn't expire) but all the sites are selling one which gives you 5 extra days but they expire... help?!