There's a good chinese just up from St. Pauls but it's a bit pricey and not quite next to Tower Bridge. The problem you have on the weekend is most things close down as we have all gone to the country for the weekend (Scotland if we can make it that far............

).
If you're looking for absolution before your evening of sin, why not try Pizza Express at Southwark Cathedral, I can recommend it. Only a short walk along the South Bank from Tower Bridge (across Tower Bridge, turn right, past HMS Belfast, under London Bridge, Southwark Cathedral on left) and the restaurant is reasonably priced and great decor inside as it is actually part of the cathedral. The area is also great for a bit of sightseeing, the original 'Clink' jail is just up on your left (this area of London used to be nothing but prostitutes and whorehouses in the middle ages; nothing's changed) and the Borough Market area adjacent to the Cathedral is a popular location of a number of famous films (Mission Impossible, Lock, Stock, & Two Smoking Barrels, etc.) so is good for a stroll round and there's a couple of good pubs close by. Plus under London Bridge is a good nightclub I'm told, but I have no idea if it's open at weekends.
If you're looking for a curry, Brick Lane is a short walk from Tower Bridge and it's not short of Indian Restaurants and you won't need a reservation. Good grub and plenty of Cobra beer.