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.