I suppose it depends whether they got them prior to service or after.
In her case and altering the uniform to cover it up, do they do an RAF hijab. plus are you seriously going to alter the uniform of every serving RAF female to simply adhere to the whims of this one person? I think not.
A bit of a case of if it was her desire to become a member of the RAF, she should have learnt the requirements before decorating her body so. Her choice and her consequences.
As for the army rules
http://www.army.mod.uk/news/26584.aspx
Hers appear not to be on the rear of her neck, but the side, so may well be inadmissible in full time service.
A change to the Queen’s regulations means that body art can now be displayed on the hands and rear of the neck – two areas that were previously banned.
Surely that applies to all forces?