My guess is that a pass won't save you money unless you're booking last-minute, which it looks like you won’t be.
Look up prices for the Eurostar on their website (August is not available yet, try again in February, but you can get an idea of pricing if you look at June or July). Three separate tickets: London-Paris, Paris-Brussels, Amsterdam-London
There’s also a cheaper slower train between Paris-Brussels. Details here
Brussels to Berlin can be as little as €28, if booked in advance. You also have a sleeper train option.
I would have a read of Seat61’s guide for London Berlin, looking at options 1, 2 and 3 here which route through Brussels.
https://www.seat61.com/Germany.htm#london-to-berlin-by-train
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If you do want to look at the rail pass option, you’ll have 5 travel days. You’ll need reservations for the Eurostar (€25-30 each), and for the cross border train from Brussels into Germany (a few € on DB, or €25 if you take Eurostar). You’d also need reservations if you take the sleeper train, and I’d recommend reservations on the German domestic trains in August, though they’re not compulsory.
https://www.seat61.com/interrail-and-eurail-reservations.htm
There’s a great summary of how the pass works here