I agree, it's a total outrage. Unfortunately the law here is much muzzier (hence need for parliamentary petition). Apparently it's legal to set different dress codes for men and women, as long as the level of 'smartness' is equivalent (and in this case - and similar cases involving airlines and cabin crew - that allows management to interpret 'smart' => painful for female staff). BBC has an article on the UK legalities here:
http://www.bbc.co.uk/news/magazine-36265545