Before they increase capacity, the powers that be would be better occupied improving the third world state of the terminals and the bloody dreadful public transport access.If I had never been to the UK before and my first impression of the country was Heathrow, I would be inclined to turn round and go home.
I agree with the postings that suggest a brand new South East mega airport with fast (and affordable to the masses not just corporate fatboys) transport links to London AND the rest of the UK.