OP, conversation went a bit like this:
Cousin who is long term M&S manager mostly on Food Hall: apparently feminists are boycotting us! FFS! Can you imagine! Purple, you're one of those bolshy types, do you know anything about this?
Me:
Cousin: That's ridiculous. As if! Purple, what are you smoking these days?
Me: alrighty. Ask your colleagues in ladies' wear.
Cousin: OMG, I thought you were winding me up. I've talked to them & wow, they are NOT happy. This isn't actually happening in my regional store though & obviously it's a bonkers London thing.
Me: I think you'll find it's national policy. Although NOT where I live overseas, interestingly.
Cousin: ah...yes, this is now suddenly massive & everyone is losing their shit about it 
. Do you still want those short dated turkey crowns?
Cousin is of the opinion that things would have been better left under the radar (apparently the occasional bloke trying on bras has been indulged forever), but now it's all anyone ever talks about in M&S meetings, with the direction of thinking being overwhelmingly that this is an AWFUL idea.
M&S managers know their customers, & how they'll feel about men trying on pants in the cubicles beside them.
Cousin says it's a mighty clusterfuck, based on rumours she hears emanating from higher up, with several stores flat out refusing to comply.
We'll see....