Heathrow to Paddington is about 15 mins by fast train or 35 mins by slow train.
Gatwick to Victoria 30-35 mins.
Luton (airport, not Parkway) to St Pancras is 35-40 mins.
Stansted to Liverpool St is 45-50 mins.
SOU to Waterloo in 66 mins seems ambitious - 70-75 mins seems more realistic
Furthermore, a fully flexible single ticket (no, I am not buying a fixed time advance purchase ticket for travel after arriving on a possibly delayed flight) from these airports to London does not cost anywhere near £43.
If SOU can get South Western rail to bring back the former rail+air ticket, then SOU might get some more London-centric pax