This is down to you - but here are some pointers:

When you have booked your transport into London Town
please then book your FREE CAB to and from the venue.

Please note:
This year FREE CABS will only be from 
London Mainline Railway and Coach Stations - 
not from Gatwick, Stansted, Heathrow or Luton airports. 
(Last year the cab budget went out of control because of these distance journeys.) 

There are good train connections from all these airports to 
London Mainline Railway and Coach Stations.
(Return train tickets are best purchased on your arrival at the airport you come into.)

Those stations are:
Victoria from Gatwick, 
Liverpool Street from Stansted,
Paddington from Heathrow,
Kings Cross Thameslink or St Pancras from Luton.