Hi there!
I live in the South of France and wish to use the remaining travel days of my Interrail Pass to go either to Belgium or the UK but I’ve already use both my inbound and outbound trips.
I wonder if there is an easy way to find the most suitable rail stations at the French border so that I can split the trip to pay the French part and use the Pass for the foreign part.
Many thanks in advance for your tips!
You need a Global Pass valid in both France and Belgium or the UK to travel on Thalys/Eurostar.
The alternative to get to Belgium would be to buy a ticket to somewhere around Tournai or Courtrai.
You cannot take the Eurostar without using an inbound day so your alternatives are ferries, Flixbus or an expensive Eurostar ticket...
Cristal clear, many thanks for your support! 
You can’t get to the UK on Eurostar if you’ve already used your inbound & outbound travel days.
But you can get to Belgium. Where are you in the South of France? You just need to get to Italy or Switzerland, then you can use your pass to travel through Germany to Belgium
The border tariff points between Belgium for regular trains are Tourcoing(fr), Blandain(fr) and Jeumont(fr). They are not located in stations. Tickets from France to those points are not available online AFAIK.
If you're using a TGV to Brussels, you either have to buy a regular ticket to Brussels or a ticket of the "Pass 3” type, whichever is cheaper. Pass 3 tickets are not available online.
If you're using Eurostar (ex-Thalys), then you need a regular ticket to Brussels.
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