From where I used to live in Greater London, to Heathrow, by cab, was about 1h 30m. Same route, using only TfL, was probably 2h. Same route, using HEX between Paddington and LHR, was about 1h15m. I have no idea what the ull opening of LizPurp will do to the transit times, but chances are it would bring them down to maybe 50 minutes.
Getting to LHR from where I was, it was always quicker to get to any of the major rail stations in central London than to either LHR or LGW, but LCY was relatively good (only really works that way if you're east of the centre though).
Getting to LHR from where I was, it was always quicker to get to any of the major rail stations in central London than to either LHR or LGW, but LCY was relatively good (only really works that way if you're east of the centre though).