Ah, right! Here's an approximate comparison then, measuring from one of the terminus stations serving central London, according to railway timetable data:

0h 30m - Heathrow

1h 00m - Gatwick

1h 00m - Luton

1h 30m - Stansted

2h 30m - Manchester

3h 00m - Birmingham

3h 00m - Bristol

4h 00m - Cardiff

Plus Channel Tunnel trains:

2h 00m - Brussels

4h 00m - Amsterdam

Looks like I was wrong about Manchester being further than Cardiff though!!