For accomodation you can check: www.booking.com, hostelworld, trivago, air bnb,… The usual websites.
Note that some long distance, high speed trains and night trains have compulsory reservations. You best look into that too, especially for France and Spain.
If you want to take the Eurostar from London to Paris or Brussels/Amsterdam, reserve that asap. All about reservations, you can find here:
Greece is quite out of the way. For a 3 weeks trip I'd forget it. There are no trains into the country too -> 2 main options :
- bus from Sofia to Thessaloniki (not on your route at all though)
- ferry from Ancona, Bari, Brindisi to Patras
As said book Eurostar out of London ASAP. What are your travel dates ? Trains around Christmas and New Year are mostly full : extremely popular route + Interrail/Eurail passholder quota (exception). There are ferries in case but the whole journey takes quite long. The one to Rotterdam is very good too.
Spain and France are also annoying with mandatory reservations on most trains. You have to book in advance so it limits your flexibility. Countries like Germany, Switzerland, Belgium, the Netherlands work really good. Italy is fine too even though high-speed trains require reservations.
Night trains could be a good way to travel on long-distances but they're very popular + often very expensive...
Have a look at :
https://www.seat61.com/how-to-use-a-eurail-pass.htm
https://www.seat61.com/interrail-and-eurail-reservations.htm
Feel free to ask further more precise questions, we'll gladly help.