How about a cross embroidered to the uniform.
Still, I can say as a Roman Catholic (who grew up going to private RC girls' schools where we did indeed wear blazers embroidered with a cross in the insignia just as per your suggestion), I also wore permanently around my neck a cross given to me by my grandmother when I was born. Or I did until I lost it -- it's currently sitting on the bottom of Lake Travis in TX. I would have been pretty irked too if a Sikh girl could wear a symbol of her faith while I'd be compelled to remove my cross at our sister school in Surrey.
So, I dunno. I'm thinking UK schools are eventually going to have to admit that sometimes they can't win for losing, and give up their attempt to shield themselves from liability on student safety issues with their banned dangling jewelry rule altogether thanks to the High Court's bangle girl ruling.