Your biggest issue will be the Eurostar. Alternatives : ferry to mainland Europe.

I can't help otherwise but I can give you this link : https://www.seat61.com/bike-by-train.htm

Very useful website.

In Switzerland you pay 14 CHF for a bike day pass or a half-fare ticket if it is cheaper. You need to add a 2 CHF bike reservation on Intercity trains.

Rules are very specific from country to country, or operator to operator. Some require a supplement, some a reservations, some both, some don’t allow it during rush hour, and on quite a lot of trains it’s not allowed (or possible). The more bike friendly a country is, the easier it usually is (So very doable in Belgium, The Netherlands, Denmark, but Germany, Austria and Switzerland are also very okay). But it’s a bit of a puzzle and usually easier on regional trains, than long distance trains.

Taking your bike on older trains it isn’t super convenient (steps, small corridors). On newer trains usually no problem as there are designated spaces.

This website has a good overview, a bit more comprehensible than Seat61 imho: https://showmethejourney.com/travel-info-and-tips/taking-a-bike-on-a-european-train/

I guess it’s not easy to travel with E-bike, right?

It works the same as with ordinary bikes, I refer to the link I placed above.

E-bikes are a bit heavier. If you’re in good shape, and don’t mind the occasional heavy lifting if you encounter an older train in a station with low platforms, it shouldn’t be a problem.

You should be able to load your batteries, when there are plugs on the train (more and more).

The biggest stations are usually accesible.