Unfortunately, as shown by the many posts in recent weeks, Eurostar has limited seats available for IR/Eurail pass holders. They are very popular and book up weeks or even months in advance. They are the only rail service across the channel and they do  not offer us in station bookings once their allocation is reserved, which is full for most, if not all, of August.

There have been reports of some available at very early or late times but they have taken some finding by others on the chat group.

It seems the only options are either to pay Eurostar a full fare or find a cheaper flight from the mainland to a UK airport and use your pass in the UK to resume your train journey. Ryanair and Jet2 are  2 that may have flights available.

There are ferries from the Caen area (Brittany Ferries I believe) but I suspect they are also very busy and expensive.

There are no 2nd class seats on that date and 1st class only on the 08.13 departure from Paris, there are also 1st class available on other services from Lille.

Only if you have a 1st class pass are these available to be used.

The previous post gave the best option, https://www.brittany-ferries.co.uk/ currently have space on the 08.30 Caen - Portsmouth for £65. There are frequent trains from Portsmouth to London.

You could also try looking for a TGV to Calais and then P &O ferries. There are also TER trains to Calais but they are slow and you sometimes need to change in Amiens. In Dover lots of trains to London but you’ll need a taxi in Dover. You’ll need to pay for the ferry as well around £40.