It seems like they deliberately made sure that the dress code excluded crosses:

The airline’s uniform code states that staff must not wear visible jewellery or other ‘adornments’ while on duty without permission from management.

It makes exceptions for Muslim and Sikh minorities by allowing them to wear hijabs and turbans.

Under rules drawn up by BA’s ‘diversity team’ and ‘uniform committee’, Sikh employees can even wear the traditional iron bangle – even though this would usually be classed as jewellery – while Muslim workers are also allowed prayer breaks during work time.

But Miss Eweida, 55, from Twickenham, insisted her cross, which is smaller than a ten pence piece, was not jewellery but an expression of her deep Christian faith.

She questioned why she was being forced to hide her religion when BA’s Muslim and Sikh workers could express theirs.

[…]

Dr Patrick Sookhdeo, international director of the Christian charity the Barnabas Fund, said: “Discrimination against Christians is commonplace in Muslim-majority contexts, such as Egypt where Nadia’s family roots are. “Now we see the same thing increasingly happening within the UK.
“Her Sikh and Muslim colleagues at BA can show their faith publicly in what they wear, but Nadia and other Christians cannot. All we are asking for is a level playing field for all faiths.”

It’s amazing to me that in a country where the monarchy is the head of the church there would be discrimination against Christians.
(Link via little green footballs)