My suggestion would be :
Take the train to London and then Eurostar to Lille - stay overnight* - several nice hotels and restaurants near the station.
Take the TGV from Lille to Marseille - it's fast and comfortable (1st class is cheap as chips) with ever-changing countryside - then either stay the night in what is an interesting and lively city, or continue the same day on the coasal train towards Nice alighting at St Raphael.
I've not been on a French sleeper train for many years but the advent of the TGV has made journeys so much faster that it's all doable in a day. We've been to/from the S of France by train 4 or 5 times in the last 8 years, both direct Eurostar and changing at Lille (just avoid the Paris change as station to station is hot and sweaty on the Metro) - a painless experience
Bookings are simple to make with trainline.eu (they bought the excellent capitaine-train website) - and I think there are old-git prices too!
*as you live in the back of beyond it may be simpler to stay overnight in London, get an early Eurostar and just change at Lille - it's only a change of platform - onto the TGV (they used to run a Eurostar service direct to Marseille but that stopped in 2020 - it may be back for summer of 2023?)