Upvote:1
Use social media to make an arrangement with someone crossing in a car.
That will be more accurate anyway, since you are wondering about the experience of cars, not buses.
The bus isn't going to work. You'll hop on with the so-called "understanding" that this one takes the tunnel... and suddenly notice they're driving to a ferry terminal! Their contractual commitment is to get you from A to B, not to follow any particular route you might find interesting. If the driver has heard (e.g. from management) that the tunnel is backed up, they may go take a ferry they have a contract with.
Upvote:7
[Note, I am taking bus and coach to be synonymous here]
There are regularly scheduled coaches which travel through the Eurotunnel for which you can buy a ticket.
As an example of one firm, FlixBus offers coaches between London and Paris. However, only a limited number of all the buses will go via the Eurotunnel - in a few minutes searching, I could only find the 23:00 on 9th July 2022 travelling via Eurotunnel.
Generally the shortest journey times are a good place to start searching, and by expanding the stops on the journey you can see if it stops at FOCA (Eurotunnel Terminal Folkestone) and CAFO (Eurotunnel Terminal Calais).
However, it would be a long journey (8h 20m) to experience a relatively short time on a Eurotunnel train. There may be other coach routes (Dover-Calais?) which are much shorter.
And as mentioned in a comment by @Willeke, I wouldn't neccessarily trust that the channel crossing method in the planned schedule would be kept to - i.e. the Eurotunnel crossing may be replaced by a ferry crossing instead.