I would only do the exit and re-entry in extremis, since the Original Poster is travelling on to a domestic destination. UK Flight Connections is usually very quick - my longest wait so far in 2012 is 11 minutes, average is 4.2 minutes, with the new Fast Track - but if it is not flowing well then it will because North Security is jammed up. In which case re-entry will either give you precisely the same problem at North Security or, more likely, you'll be sent down to South Security in order to walk all the way back to the Manchester gate at the North end.

One hour is a doddle. Frankly I think MCT should be safely cut to 45 minutes and I've done a Gate C to Gate A connection quite comfortably in 20 minutes.