Employment law is a funny one and most often more and more weighted in favour of the large employer.
The exchange of labour and capital is but one relationship we all have. Most other relationships we have over a long period are far more important.
Too many bow down to "the boss" in that relationship and accept regular abuse.
I do believe an employee should do what they agree to in their employment contract. But I believe a "job interview" should be 2 way, and the employer be quizzed to the same degree as the employee. We know that doesn't happen in reality, so how do people stand up for their own individual beliefs, religious or otherwise ?
In the end if M&S can't get the staff they need, they'll have to bend, and I guess this is what has happened here. More please. I'm sure they have plenty of other staff who will fill in if necessary. No big deal.