Our first Rick Steve's Tour in May! My husband and I are looking for past experience recommendations getting from Heathrow Terminal 5 to Canterbury? Our preference would be via train and not private car.
Tube to Victoria or St Pancras and a train from there. Previous discussion: https://community.ricksteves.com/travel-forum/england/travel-from-heathrow-to-canterbury-train
You can work this out yourself and be more confident when you are doing the trip.
For all about trains, everywhere in Europe and often elsewhere, look at www.seat61.com
Google Maps offers some plotting assistance.
Don't count out a bus ("coach", as the English say) as shown on https://www.nationalexpress.com/en/help/coach-stations/heathrow-central-bus-station
I assume you are on the Villages of South England Tour?
We took this tour in 2017, and were looking for ways to get from London Gatwick to Canterbury. In the end we decided to hire a private car. The cost was only a little bit more than two walk-up train tickets, and the driver dropped us off at the hotel in Canterbury.