Flight
For those flying in from the US, London Heathrow is the best airport and is a 1.5 hours by car
Train
There is a train from London Paddington to Kemble Station (1 hour 10 minutes) and it is a 7 mile journey to Tetbury there should be taxis at the station and there are buses
Rental Car
A rental car (for any Americans brave enough to drive on the opposite side of the road!) is your most efficient option and best if you want to explore more of the Cotswolds outside of the wedding activities.