You can't scan anything, the pass QR code does not work at gates/readers in the UK. It doesn't mean you don't have a valid ticket of course.

Staff should simply let you out but they probably don't know exactly what Interrail is...

Eurail Pass is valid on Overground but not Underground, Kew Gardens has both but the station is staffed by Underground, it is not unknown for them to be ignorant and obstructive to passengers with unusual tickets.

Sooo...pay to get back to London and get a refund? Kew Gardens is amazing. I found spring!

Just approach staff and say you ticket for the Overground doesn't work on the gates, and ask them to open a gate. Only buy a ticket if they refuse.

Yeah. That's how it worked out. They let me through but griped about it. At Euston Station both ways there were turn styles open so I just walked through.

Thanks for everyone's help.