Eurostar is a great way to cross the Channel. You should book it well in advance though : extremely popular and passholder quota.

You could take a one-way flight and Eurostar the other way around (but then you have luggage limitations and so on).

About routes : there are so many so it's hard to recommend. I love the Bernina railway between Switzerland and Italy : the best way to cross the Alps in my opinion. More: https://www.seat61.com/trains-and-routes/bernina-express.htm

Night trains are a convenient way to travel but they tend to be expensive. Also delays are frequent so don't plan tight connections the following morning.

Have a look at this card : https://nachtzugkarte.de/en/

Brussels - Vienna or Stuttgart - Zagreb/Rijeka could be good. :)

I might suggest something like London - Brussels - Vienna - Slovenia - Lake Como - Bernina Railway - Strasbourg - Lille - London (roughly). I'd avoid Paris with the Olympics.

General links:

https://www.seat61.com/interrail-and-eurail-reservations.htm

https://www.seat61.com/how-to-use-an-interrail-pass.htm

I agree - Eurostar is a great way to travel, you do need to pay an extra €30 to use it. Don’t forget - your connection from anywhere in the UK to St Pancras is included. You can travel on two of your 7 days in the UK.

My general advice when using a pass would be to minimise travel in Spain/ France/ Italy, because they have reservation fees for most of the trains you will want to use. You don’t have to completely avoid, but if you were focusing on these countries I’d advise against a pass. Maximise travel in UK, Benelux, Germany, Austria, Switzerland - mostly no reservation fees here. The Seat61 link that @thibcabe gave you has details on what the reservations cost, and how to make them.

In 3 weeks, you could go down through France or Germany, to reach Switzerland and Italy (via the Bernina Pass, as suggested). Cross the Adriatic by ferry (paid) to Split or Dubrovnik, then back through Croatia (bus or ferry at first) and Slovenia (don’t miss Ljubljana and Lake Bled) to reach Austria. I would try to include Czech Republic and Berlin if you can, then perhaps the sleeper from Berlin to Brussels and Eurostar home.