UK is included in the Global Pass. As I assume that you plan to use the Eurostar my advice is that you, if you plan to travel during July or August and if you have a fixed travel plan, first make the Eurostar reservation before you buy your pass. Or you should at least check the availability of pass holder seats.

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

The UK is in the global pass. It is also in Europe :)

And brits have to buy INTERrail- NOT EUrail. Has nothing to do with what UK has stepped out of.

These passes give you 2 days to use ALSO in home country.