Validity is a month from your first travel date, so for example 31 Aug to 30 Sep.
There is no restriction on where or how much you travel.
You will need a reservation for the London - Brussels or Amsterdam portion.
Use https://www.b-europe.com/EN/Booking/Pass#TravelWish to see availability and book.
The only suitable train with availability on that date that gives enough time to connect to Berlin is the 08.16, you should book this as soon as possible as Eurostar have a quota for passholders that sells out.
08.16 London St Pancras - Brussels Midi 11.12
12.25 Brussels Midi - Koln Hbf 14.15
14.36 Koln Hbf - Berlin Hbf 19.57
would be the best option, there are later connections at both Brussels and Koln if those are missed.
Only the Eurostar from London requires reservations but you may want to get them for some of the other longer journeys.
Bahn.com for German trains, just search seat only.
For UK reservations set up an account on gwr.com and then you can make free seat reservations.
Wonderful, thanks very much, that is all really helpful information. I’ll book my pass and reserve the Eurostar segment today.