I do not go to Central London very often, there are always so many people, especially if you go to the more popular places like Oxford Street, Piccadilly Circus, Trafalgar Square and The South Bank. You almost have to queue up just to take a photo (by the way queuing is a popular way to waste time for many Londoners).
But of course these places are almost unavoidable, if you want to go shopping then Oxford Street is the place. If you want to watch tourists or visit Soho then Piccadilly Circus or Trafalgar Square are good starting points. And The South Bank is always worth a visit.