Ah sorry guys, yes there is some useful links. The SFR Customer Service is hard to get hold of but you can use this link to send them the online form, there is no direct email you can write to: https://assistance.sfr.fr/contacter/#nav=TM-5-5-6
The mediator said that one should only contact them if there is really no help from the telecomcompany so first step would be to reach out to the customer service via email with the link above and then if they don’t respond favourably hit up the mediator on: https://www.mediation-telecom.org
then you can form your complaint.
There is also an update from RATP. I got the letter from DGCCRF and they said that there is a mediator for the public transport as well: https://www.ratp.fr/mediateur/
Basically you need to send the customer service an email and have in writing that they can’t add your GB IBAN for the DIRECT DEBIT. I then reached out to the mediator and got an email today saying that my claim is valid and that RATP will go into mediation