Like I say, good luck to the FA or FIFA with the court case if they actually tried to force someone to "work" for a national team, especially an away fixture so you can add human trafficking to the slavery chargeThere is. FIFA have the power to suspend any player that refuses a call-up, these threats were fairly common place back in the day. For example I can remember Makelele being threatened with a suspension when he tried to retire from International football - link.
I guess answering my own question, it's up to the FA to push the issue and only then will FIFA act and in this case they haven't.
What they can, and do, do is say if you declare yourself injured you can't play for your club side for a period around the international fixtures.
So, for example, Liverpool couldn't claim Salah was injured for Afcon but then play him in the FA Cup third round