Unfortunately the pass holder seats on the Eurostar are sold out on most departures during July. 

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Could you change your flight so that you fly to mainland Europe?

Being dependent on a reservation on the Eurostar or any other means of transport over the Channel is a bit risky when you try to make your reservations so late.

Your options to get from London to Paris are:

1 Pay for Eurostar separately, it will cost at least €200

2 Fly

3 Bus, overnight takes about 10 hours

4 Train-Ferry-Train (multiple) again overnight

or

5 Change your plans

If you want details on any of those ask.

@4; Currently NOT possible-and in the times it was, it was only in mid-day (for the ferry), not overnight. That is for the usual short crossing Dover-Calais.

There are also a very few other longer ferry trips-to Cherbourg-but chances quite high these are also fully booked for next weeks

Or has this changed again-the 2 competing ferries refused to take single foot-pax.

Flutes: cheap flites -relative-prices are also quite high-ar by RYANair (a very bad airline, but ultra-cheap-DO read all rules very carefully!!0 and U2=EasyJet.

Airports are overcrowded and have extreme long waits for security. Many airlines ahve cancelled many planned flites-also due to staff shortages.

BUS; FLIX or blabla.

Ferries that Currently take foot passengers UK - Mainland Europe

DFDS Newcastle - Amsterdam 1 overnight sailing

Stena Line Harwich - Hook of Holland 2 saiings 1 day, 1 night

P&O Dover - Calais currently only offering foot passenger on a handful of morning/afternoon sailings (before their self-inflicted problems they took foot passengers on all daytime sailings but not night)

DFDS Newhaven - Dieppe 3 sailings daily

Brittany Ferries Portsmouth - Caen, Portsmouth - St Malo, Portsmouth - Cherbourg, Poole - Cherbourg, Plymouth  Roscoff.

Can you reccommend me the bus company from London to Brussels?

Is it safe ?

Do I have to reseve seats or its ok  with the interrail passes?

I'd suggest to google a bit and check reviews.

No bus on this route is included in Interrail. You'll have to book a separate ticket.

Thank you for your kind reply. I have not thought so. Alternatively, I will be taking a flight outbound. Luckily I can still reserve a seat from belgium for inbound. But i am still conscious of the inter-regional trip& especially in crossing to other countries. Eurail website just mention ready to board@ reservation can be done but not required. However in reality, i dont know how flooded the train would be. 

Thank you for your reply. I bought a flight to geneva from london. I hope everything will be smooth despite the overcrowded europe.

Thank you. i take a flight to geneva instead & skip paris.Swiss is our main intention so at least we can enter europe.