You can definitely travel in the UK and Eurostar. The only query is whether you’re limited to two days or not.

Interrail makes a bit of a hash of this. The T&C’s refer to UK as a participating country (and they don’t mention IoM at all). But in reality the rail territory is GB, not UK.

If they’re allowing you to select IoM as your country of residence, then I’m almost certain you’ll be allowed unlimited travel in the UK, including Eurostar.