Advance tickets are somewhat cheap in advance (esp. off-season like Sept-Oct) but then you are tied to a specific connection.

I’d buy a Interrail pass for the number of days you might need (you can buy it at any time). It’s a bit cheaper on https://www.myinterrail.co.uk/interrail-passes/global-pass/

You’ll have a lot of flexibility with the pass as you can basically board any train you’d like. It also includes your journey through the UK to your home (the pass is valid on 2 days of your pass in your country of residence).

You need a seat reservation for the Eurostar (30€) which can sell out quite in advance (limited number of seats), this would be the only obstacle to your trip. But then you could go via Berlin, Amsterdam, Brussels or Prague and Munich or… Seat reservations are optional in Germany, Austria and not possible in Belgium or the Netherlands. Often required in Poland but for a small fee (0.1€ I believe).

The Eurostar between Brussels and London can be pretty expensive without a pass (at the moment, 57£ but that’s 5 months ahead)

thank you so much. very helpful. what you say oct is best as its least busy or no? and i would deffo have to book eurostar when i get the pass is it? 

another Q is the first class pass worth it? do you get the bigger seats with the pass in eurostar ? thank you 

October should be less busy than summer for sure, but surely similar to end of September. Holidays are always exceptions. (occupation should be no problem in 1st class anyway)

I’d say that a 1st class pass is worth it, the price difference isn’t as large as for regular tickets. You’ll enjoy emptier and quieter carriages and bigger seats (Eurostar, German ICE,...)

Optional reservations (in Germany, Austria, Czechia,...) are also free of charge with a 1st class pass using tickets.oebb.at (add Interrail as a discount and select one-way tickets)

The only difference : Eurostar Standard Premier seat reservations cost 38€ (while 2nd class is 30€). You do not need to book Eurostar right now, don’t worry, but I wouldn’t wait either until a week before travel date

thats really helpful info, thank you