You are allowed to take as many trains as you like, in your country of residence to leave the country.

So it's just one (outbound) jouney. But you need to do both journeys the same day. If you'd stay overnight in London, it would count as two journeys in your homecountry.

Thank You for your prompt response. Our possible issue is that we live in Penzance and were hoping to get the night train to London. It’s difficult to do this trip in a single day. Our overnight would be on the train from Cornwall.

How do we get a definitive answer or solution.

By using the Night Riviera (not running many nights January to March) you'd need to use both travel days in your homecountry. 

It's possible, but you'll have no travel day in your homecountry left to get back.

Solution: Buy a normal ticket for the Eurostar from London or a normal ticket for the Night Riviera (maybe just for the first station past midnight) or use just day trains.

Then you'd use your 2 inbound/outbound days and would need regular tickets to come back or for either day.

You could buy a regular ticket as far as Bodmin Parkway and then start the pass there as the night train leaves after midnight (CET).

But it's really easy to do Penzance - London - Paris in a day : hourly GWR train, Tube between Paddington and St. Pancras and then passport controls before Eurostar. You can leave Penzance in the afternoon.

Another option would be the overnight ferry from Plymouth to Roscoff (from 126€ for 2 people).