France round trip

You need a travel day for every day on which you plan to take a train with your pass. The global Pass allows you to use trains in your home country on 2 of those days.

As long as in both cases you travel from preston to London and continue to Paris on the same day you can do both those journeys with the Pass.

Other important things: Eurostar reservation have a limited amount of passholder reservation and those often sell out, so its important to book those reservations early. 

Booking the reservations for highspeed trains france early also makes sense as there is a limited amount of 10€ reservations before the reservation price increases to 20€. 

So i assume you plan to do it like this with the travel days ( As Preston → Paris has to be done in 1 day):

day 1 Preston → Paris (via London)

Day 2 Paris → Lyon

Day 3 Lyon → Nantes

Day 4 Nantes → Paris

Day 5 Paris → Preston (via London)

The cheapest place to book interrail reservations online for france is allaboard.eu as they only charge a booking fee of 1€ on top of the reservation price (compared to interrail and raileurope which charge 2€)

With the eurostar and france you will unfortunatly need quite a few expensive reservations. As all highspeed trains in france require a mandatory reservation.