Yes, they're mandatory. You won't get past the security and border controls without one.

Reservations are also limited on Eurostar services (passholder quota). Trains are almost completely sold out until early January.

What would be your travel date? Alternatively there are Flixbus from Belgium/the Netherlands to London or Stena Line ferry from Hoek van Holland to Harwich. The latter is esp. convenient as you can spend the night onboard. Neither are cheaper than Eurostar though.

If you mention the travel dates we'll try to help.

Thanks for the reply! 
my travel date would be friday the 27th of this month. So in a few days. 
I didn’t know there was a ferry between Hoek van Holland and Harwich so this could be my live saver. 
 

I think this option is cheaper though, because the cheapest 2nd class travel with Eurostar is €220 on my chosen date. This is a one-way ticket.
The one-way ticket with the ferry however is €80.

Yeah I was speaking about Eurostar reservations (30€ with Interrail) vs ferry tickets.

Eurostar availability looks like this out of Brussels:

- 26th December: 17:56, 20:56

- 27th December: 20:56

Stena Line offers a little discount for Interrail travellers (discount code EURAIL2024) so the ticket would cost 61.40€.

You would arrive in London around the same time as the 20:56 Eurostar so really choose which option is more fun. Perhaps ferry to the UK and train back! ;)

(Or you could get the overnight ferry for 130€ incl. cabin and no need to bother with accommodation in London!).