After heavy rain all weekend there is widespread disruption today on rail travel across the South East of England (including all services to Gatwick Airport and to/from Victoria) and on the London tube network, due to flooding.
The issues are too numerous to note, and this is a live and changing situation, but just to be aware and to take local advice on other routes (which may include buses).
The Overground lines to Watford Junction are affected at the time of writing (8.20am) (the slow trains) but the fast lines are open- so it shouldn't affect people on this forum.