Jumping...

The 78 was of course a double decker bus in the past - but it appears not to have headed into Nunhead then.  Online there has been some discussion about whether the route will change in Nunhead with the move back to big buses (because of the low bridge), but apparently (thankfully) not.  The route will stay as it is.  Useful and interesting.

Indeed, according to Wikipedia:

In December 1952, a number 78 double-decker bus was crossing Tower Bridge. At that time, the gateman would ring a warning bell and close the gates when the bridge was clear before the watchman ordered the lift. The process failed while a relief watchman was on duty. The bus was near the edge of the south bascule when it started to rise; driver Albert Gunter made a split-second decision to accelerate the bus, clearing a six-foot drop onto the north bascule, which had not started to rise. The conductor broke his leg, and twelve of the twenty passengers aboard received minor injuries. The driver was later rewarded with a £10 bonus.

Wikipedia also provides a link to the news story that Time magazine carried on the same event.

So clearly, in addition holding more passengers, the 78 may well bring a new sense of exhilaration when it crosses the Thames...