If you only will travel from London to Toulon and back, and you plan well in advance I would think that single tickets will be cheaper than a pass. You need to calculate.

The main issue is the Eurostar. The reservation of a pass holder seat in 2nd class is 30 EUR and the cheapest single ticket is about 40 EUR.

If you find a cheap single Eurostar ticket I can't imagine that a pass would be worth the price. 

Here you can check the prices for the Eurostar:

https://www.b-europe.com/EN/Trains/Eurostar

Fly RYAN to Marseille-then train is likely far cheaper
there is also again an overnite Prais (Austerlitz)-Toulon (onward to Nice)-off season he seats (mostly old 1s cl cars) can be quite cheap. It also has couchette=lie down bunk.