For any transfer between London airports allow at least 4 hours between landing & take off if

a) not landing at Heathrow or Gatwick
b) not transferring during rush hour

In those cases I'd be allowing at least 5 hours

In most cases the direct coach between airports is the fastest option - the one exception is between Gatwick & Stansted which goes via Heathrow and takes substantially longer than taking a train to London then another train out of London EVEN with a Tube transfer