Let me add some suggestions here... :)
A personal favorite of mine (though I'm not alone) is the Bernina railway in Switzerland. You could do something like:
- UK - Paris - Strasbourg (overnight) -> expensive high-speed trains, must be booked ahead of time
- Strasbourg - Basel - Zurich - Chur or somewhere in Engadin (little hike maybe) -> very frequent trains, all reservation-free. Motto: go with the flow.
- Chur/Engadin - Tirano : the trip's highlight aka the Bernina Railway! In summer there are open-air wooden carriages, regular carriages with pull-down windows or the touristy Bernina Express.
- same day continue towards Lake Como and Milan: a replacement bus will be involved (upgrading works for the 2026 Olympics) but don't worry about it, it's well setup.
- Northern Italy - Trieste - Ljubljana: rather than Milan perhaps stay for a night or two in either Verona, Bergamo, etc.
- Ljubljana - Zurich or Stuttgart night train: very convenient and usually not too expensive. Book a private double sleeper ahead of time.
- Stuttgart - Paris - UK: you could head home straight after the night train. Do allow for a 1-2h delay into Stuttgart.
Perhaps this wasn't enough Central European. ;)
Alternatively something like:
- London - Brussels Eurostar followed by the Brussels - Prague night train run by European Sleeper (not daily). Waking up along the Elbe is really cool.
- Prague - Brno (to avoid visiting only capitals!)
- Brno - Tatra mountains: some nice scenery there and cute little trains
- Tatra - Budapest
- Budapest - Vienna - Graz
- Graz - Zurich: long-distance EuroCity across the Alps. There's a 1st class panorama carriage.
- Zurich - ... - home
(Budapest - Zurich/Stuttgart night train exists as well.)
About option B with Finland: the obvious idea would be a loop around the Baltic Sea. I'm not a big expert on the Baltics but I know international train connections have recently gotten better.
The Hamburg - Stockholm night trains (SJ and Snälltåget) are extremely convenient so I'd recommend them. Book ahead of time. Same for Stockholm - up north.
We always recommend the Man in Seat 61 website for much helpful advice, for example: https://www.seat61.com/interrail-and-eurail-reservations.htm