Train or bus Bath to Heathrow?

For the London to Canterbury train, I would download the app for Southeastern Rail and buy your tickets on there. It does not look like there is an advantage to buying this route early as the prices look similar for day after tomorrow to late August.

Is the tour staying at the Canterbury Cathedral Lodge Hotel? The Canterbury EAST train station is a teeny bit closer than the Canterbury WEST station. I took the train from Victoria Station to Canterbury EAST and had an easy walk to the tour hotel (which was not the Cathedral Lodge but was nearby).

You can also look at the trains to Canterbury West. Some require a change of train, some are direct.

For your departure from Bath, I would probably take the National Express coach to Heathrow and then take the local shuttle to your hotel. What time does your flight leave? Asking because you might be able to easily stay in London that last night.

London St Pancras to Canterbury train- if you buy now for an Advance (train specific) ticket it should be around £16.

Buy on the day, as a walk up fare it is between £43 and £50 depending on the time of day., higher still in morning and evening peaks.

National Express run every 2 hours from London Victoria- book now and the fare is about £7- it will go up. I have a Senior Coachcard so it is £4.50 for me (33% off). The card costs £17, a year so isn't usually worth it to someone from overseas.

The fare tomorrow is about £15.

The fare on Southeast trains from London Victoria to Canterbury East is 14.40 GBP for August if you buy now. If you were buying for day after tomorrow it would be 16.20GBP. This train is direct and takes 1h30m.

Do you know where you are going to stay in London? You might choose based on which departure train station is closer to your hotel - Victoria Station or St. Pancras International.

I took National Express bus Bath to Heathrow. It works well.

You will enjoy this tour.

Flixbus also run from Bath to Heathrow, at very similar times to National Express , and are a lot cheaper normally. They have started since the last RS guidebook was published, so are not mentioned.

Just like National Express the fare is dynamic, and in theory could sell out, but is unlikely to, more than a day or so ahead of time.
Of course I will always favour National Express over Flixbus, but it's good to know of the choice. I personally think Nat Ex are a better product, better seats and often more leg room.
Nat Ex drivers assist with your luggage, flix bus rarely do. On them you put it in and retrieve from the luggage hold as a general rule.
The £16 rail fare from Victoria to Canterbury is a capacity controlled Advance (train specific) fare. No one knows if it will sell out on a specific service or day. The walk up fare is a lot higher, as are all Advance fares.

Thanks for the helpful information, everyone. It helps to benefit from the experience of others!

N Scott thanks for your question. We are doing the Sept 7 tour. Our 10th RS. our Plan is two nights in London in the at the Washington Mayfair after a red eye from Seattle. (No rooms at the Canterbury Cathedral)
back to your question. We have decided to take the morning train: Bath Spa to Paddington and the Piccadilly to Heathrow for
the 9 hr flight home. We enjoyed the ride when we did the BOE tour.
Slightly more expensive but more comfortable than the bus.

From Paddington:

If you book Heathrow Express in advance, you can tickets that cost £10 instead of £25.

The Elizabeth Line from Paddington is £15.50

Both are better IMHO than the Piccadilly Line.

I am in Bath now and headed to the airport tomorrow, Heathrow. I opted for the bus as it’s direct and much less expensive on a Friday morning than the train by a substantial amount. I also didn’t want to have to switch trains and deal with the my luggage.