Yes, you can use trains in the UK. Be aware you’ll need a reservation fee on the Eurostar (€30). You can do the rest of the trip without compulsory reservations, depending on the route you take.

Seat61 has a some suggestions for the routes you can take, including sleeper trains if you wish.

https://www.seat61.com/Germany.htm#london-to-stuttgart-and-munich-by-train

Option 2 (ICE to Cologne/ Frankfurt then Brussels. Eurostar to London. LNER or Avanti to Glasgow) will be the one that only has one compulsory reservation, on the Eurostar.

If the background of your question is to find out if traveling on an Interrail ticket is more economical than buying your own tickets, then I would think that Interrail is certainly cheaper considering the high cost of UK rail tickets. They do have more economical ‘advance’ fares, but then you really need to fix your entire journey well in advance, and stick to it.   

If you want to go off the beaten track, avoid the Eurostar and book a pedestrian ticket on ferry instead, e.g. Dieppe-Newhaven (railway stations at both ends), Calais-Dover (railway stations at both ends) or Hoek van Holland-Harwich (railway station at the UK-end and a non-Interrail metro at the Dutch end).