I just had a look at the website of B europe, where you can book interrail fares for Eurostar online.
It seems that there are no more places available for June 24th.

There are still regular tickets available, so no pass fare.
These prices are very expensive, starting at 241 euros.

As @Sven  says t looks like all seats are sold for Railpassholders

There are strikes around that day in GB-which means many people have had to reschedule their trips. 

I checked earlier today and then the earliest pass holder seats from London to Paris were in the evening of the 2nd of July.