Yes of course -those on that island in the north sea are still Europeans-and must hence buy INTERrail (it has nothing to do with EU-notwithstanding the name-it was there decennia before there was even an EU) Turkey, Serbije, Moldova are also EUropean and can buy the pass.
Global passes ONLY can also be used-with a hefty supplmt for the privilege and also a small quota of such seats- on€*-long advance booking strongly advised in hi-season, This pass is ALSO valid on only 2 days to get out of GB and in at the and-to whatever local station (it may even be Wick on the far north of Scotty). DO note that FRance for its famous TGV also is a hard nut to crack -again supplmt and sometimes hard to get REServ always needed-and ESpana is even worse. DO plan better and not just cart off a few of those long hi-speed tracks. EUrail is valid unlimited anywhere-hence not for sale to Brits doing line bashing on what once was British Rail
In GB there are also a few private trainticket agents also able to sell these passes-some even still if so wished the old-style paper pass.
Please be aware that the pass holder seats on the Eurostar on popular departures can sell out weeks and some times months in advance.