I have a 4 day pass that I will be using 3 days of for a trip from the UK to Spain - and flying home.
Is it possible to use the final day on the Interrail ticket, within the month, when we have returned, to travel from Manchester to London? Or must I travel out of the UK?
I’d like to use it in one day if possible.
As long as you only used one day in the UK on your outbound trip, then you can use your final day to travel from your port of entry (airport, ferry port, or Eurostar) to anywhere you like.
Strictly, that's the only journey you're supposed to make, I think. So you couldn't use it several days after you arrive home for a completely different journey.
I'm not sure what the official wording is though.
If you still have an unused inbound/outbound day you can use it whenever you please within the validity of the pass.
It is allowed but some picky ticket inspectors may cause issues (especially in the UK). If that's the case tell them that you're coming from/going to x airport/port which is explicitly fine to do.
Here's the terms and conditions. Section 5.2 governs travel in your country of residence. You can choose your own interpretation!
https://www.interrail.eu/content/dam/pdfs/terms-and-conditions/CoU%20v.9%20-%20Clean.pdf
Hmm, it doesn’t really feel appropriate according to the rules. Whistle stop tour to Paris and back in a day? 
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