Yes it is possible, just use your travel day and a outbound. You can only use Interrail in your country to get from your home to the border or airport and one time back. This is the outbound/inbound rule.  

Travel in your own country | Interrail.eu

Hi it still did not answer my question.

I will ask it in this way

Does London St Pancras International count as the border? So if we travel from London St Pancras to Luxembourg will I be ok to use 1 of my 10 tickets? 

Interrail does not work with tickets but with travel days. You can travel as much as you want on each of those days. On 2 of those days, you can use Interrail in your country of residence (inbound/outbound journeys).

London is not the border. You will use an in/outbound journey when using Eurostar with an Interrail reservation. But if you're starting in Manchester on the same day, you will already have an active in/outbound journey so that will also be valid for Eurostar.

Thank you so much for this really helpful reply, much appreciated 

David

I’m planning to use Interrail for holidays in Europe later this year. While doing research on the www, I read that Interrail passes were no longer valid in the UK. I also read that the Interrail pass is valid in the UK for my outbound travel day. Which is correct? I live in the North of Scotland and it would be nice if I could travel outbound from home to Paris.

@philpq  Interrail is still valid in UK. ATOC announced to leave Eurail and Interrail but just one day later they said they wanna stay. Means nothing changed :)

Yes you can use your Outbound journey to get from North Scotland to Paris (note that Eurostar needs a reservation)

Thanks, that’s great news.