What you buy is up to you. Compare the prices of the various options.

Also I'm wondering why you would buy a Britrail pass if you're already planning to buy a Eurail pass. It might be cheaper to buy a continuous Eurail pass for the whole period.

I looked at different options, and it is cheaper for me to get a two-month pass, rather than two separate 15 day passes. I won’t worry about not using the pass for two weeks while on the ship.

Eurail Pass doesn’t cover Great Britain, except for the Eurostar train from Paris to London. Great Britain has their own pass.

The Eurail pass does cover Great Britain, including all domestic trains. 

If you're not travelling every day, a 15 days in 2 months pass might also suit you. That gives you the flexibility to travel 15 days spread over a period of two months (60ish days).

Thanks for the replies. I now see that Britain is included, but I remember at one point it wasn’t, so that solves a lot of problems. Looks like the two-month Global Pass is best for our needs.

Thanks everyone for your replies.