Note that (mandatory) seat reservations will be a significant part of your budget on top of the pass :
- 30€ Eurostar (passholder quota too -> book ASAP)
- 35€ direct Paris - Barcelona TGV
There are ways to save money like crossing the Spanish border on regional trains or taking a night train from Paris to the Spanish border*. In any case do not leave it to book during this busy period.
*this would be really convenient as you could arrive in Paris at 6pm, have dinner and then board straight to the Pyrénées or the Mediterannean.
General guide : https://www.seat61.com/interrail-and-eurail-reservations.htm
Questions welcome.
I had a quick look and Preston - London will be best done via Leeds. You could arrive as early at 10am but then you'd have to wait anyway as the first available train is the 14:31 service.
Leave Preston at 07:44 or 08:44, make your way to Paris, have dinner there and spend a night in the city or board the night train right away! Pretty ideal :)