There is a limited quota for passholders on Eurostar trains. Seat reservations can sell out weeks in advance, esp. on holiday weekends like yours. There is no availability on 26th May, either to Paris, Lille or Brussels
Check availability here : https://www.b-europe.com/EN/Booking/Pass#TravelWish
Options :
- train to Dover, ferry to Calais 30£ and then train to Paris (a full day of travel)
- ferry via Southampton or Portsmouth
- Flixbus
- or very expensive 200€+ Eurostar tickets…
- travel another day
3rd June :
First Eurostar of the day has availability, departure 07:13 from Paris. Otherwise go to Lille (regional train or TGV for 10-20€) and then there are more trains to London. Eurostar costs 30€. In any case book ASAP
Advice : https://www.seat61.com/interrail-and-eurail-passes-on-eurostar.htm