Crossing the Channel as a foot passenger is indeed quite complicated now. There are 2 main options : Calais - Dover (the train station is far away) or Dieppe - Newhaven. Also possible further west from Caen or Cherbourg. See : https://www.seat61.com/trains-and-routes/london-to-paris-by-ferry.htm

There are Flixbuses across the Channel but you'd have to go all the way from Lille, Paris or Bruges to London, not really a fun experience in my opinion.

If you don't have to travel from France I'd really recommend the Hoek van Holland - Harwich ferry : daytime or overnight (30% Interrail pass discount). 30 min subway from Rotterdam and then 1h15 train to London. You have cabins for the overnight ferry crossing. More info here : https://www.seat61.com/trains-and-routes/london-to-amsterdam-by-ferry.htm

I've done that journey 2 weeks ago after a month in the UK from Switzerland : one way Paris - London Eurostar and return by ferry to the Netherlands. Feel free to ask questions about it :)

Do you have to do the whole journey in a day ? You can get to Rotterdam easily by daytime trains and then either take the overnight ferry or stop for a night and travel the following day.

Thank you very much, this helps.

I'll have a look to your suggestions. My mother is travelling with us, and I can't take her into the Eurotunnel. She is afraid. Thus looking for ferries, but it is really complicated. Soon we will travel separately.

That's funny, that you just took this trip! Hope you had a good time.

We plan Canterbury, Edinburgh, London, Brighton.

You can tell her that the tunnel is really safe and way safer than a bus. It's not different than any other rail tunnel in Switzerland or elsewhere : only darkness for 20 min. :)

I'd say that the ferry from Rotterdam is a good choice but it's doable from France too. Feel free to come back, the pages I linked are exhaustive and good advice !

Yes my trip was fantastic. Once you've crossed the Channel British trains are easy to use and you don't need seat reservations -> they're recommended (but free) on mainlines like London - Edinburgh.

You could also consider the Rotterdam to Hull overnight ferry. The station in Hull is a couple of miles from the ferry , there are connections, and then trains to York (for Edinburgh) or direct to London with Hull Trains. This is operated by P and O and allows foot passengers.