You can check here without buying a pass:
https://www.b-europe.com/EN/Booking/Pass#TravelWish

Pass holder seats are limited, so availability on the Eurostar website for normal tickets doesn't necessarily mean there are still pass holder seats.

If you don't find seats London to Paris, look for London to Lille instead; there are many other trains from Lille to Paris.

If you search morning trains you won't find anything but keep pressing later trains and you'll find availability on the late afternoon trains to Paris or Lille.

Do not wait to book but at least you'll make it to Paris :)

4€ booking fee per order through the link given by rvdborgt or 2€ per person per train on interrail.eu

Btw to save a few £ you can buy the pass on myinterrail.co.uk

Hi Thibcabe

many thanks for your response. I’ve just gone into myinterrail.co.uk which seems to be much cheaper as I’m travelling with a child aged 11 who appears to be free.!! The only thing I’m confused about is they seem to send your pass through the post as opposed to sending an online link - do you think this will hold me back in anyway? will I need to wait for it to come through the post to book the Eurostar? I’m just a bit worried as I can see some of the times are selling out . And I still don’t know if I can book the reservations on these passes on the 24th July! Sorry to bother you again!! My head is spinning! 

Children under 12 y.o. also get the free pass through interrail.eu ! There’s no such big difference, only a few £. Please note that children also need to pay for seat reservations (same price as adults).

You can get Mobile Passes on myinterrail.co.uk, you’re able to choose during the booking process :)

To book on b-europe they ask for a pass cover number. Either you generate one here or put I11111111 (the number doesn’t appear anywhere) : https://www.interrail.eu/en/book-reservations#/generatePassCoverNumber

You don’t need to have bought a pass first. I’d definitely book Eurostar ASAP.