A pedantic point to start with: The discount provided (in the UK) by a Senior Railcard is 34%, not 25%.
Next, a (UK) Senior Railcard is provided by the Association of Train Operating Companies (ATOC), which means that it's
only valid on the services of those companies. You can't use it abroad (or even for travel on Eurostar).
In France you need to use SNCF's own 'Carte Senior+' for discounted travel on TGV, TER and Intercités services:
http://www.sncf.com/en/discounts/senior-plus-card
On other services you don't need any form of railcard, as there are automatic discounts for those over 60 years:
http://help.uk.voyages-sncf.com/en/train-fares-seniors
Lille Flandres (and Lille CHR) to Lens services are operated by TER trains (so you don't need a railcard to get a concessionary fare). Some slower services from Lille Frandres to Arras are operated by TER trains but the faster ones (and those from Lille Europe) use TGV trains, meaning that you can't get a discount without a Carte Senior+.
So:
(a) you can keep your Senior Railcard in your purse; it's not valid in France ; and
(b) you should check which type of train you'll be using in order to find out whether you can pay a concessionary fare.