I would highly recommend the Churchill Ladies Pilot Shirt. I wear the short sleeved version - haven't tried the long sleeved one. The quality and fit are very good and they still look white after repeated washing (chucked into the washing machine).
Hope this isn't construed as an advert:
Burlington Uniforms Ltd - Providing all your staff/work uniforms and corporatewear needs. The code for the ladies shirt is 5212.