Please add information about date and time. Without that information it is impossible to give advice. 

May 20 at 9:31

thank you

All trains London to Paris are sold out that day for passholder.

There's just availability one day earlier or later or May 20 to Bruxelles at 7.04. 

Indeed take the Eurostar to Lille/Brussels. Book ASAP.

Itinerary would look like this:

- Eurostar London St. Pancras - Lille-Europe 07:04 - 09:26 30€

- walk across the square, have some breakfast

- TGV Lille-Flandres - Paris-Nord 10:42 - 11:44 10€

Eurostar trains are extremely popular and there's a passholder quota. On busy dates (yours is a holiday weekend) trains can sell out weeks in advance.

As I said book the remaining Eurostar ASAP (today). Otherwise you'll have to buy a full fare ticket or look at other less convenient options.

Best website to book these trains is www.raileurope.com (desktop website), since there you don't have any extra booking fees.