There is nothing on the National Blood Service website about not being able to donate platelets if you are a women, it is, however, quite a commitment and not everyone is suitable, see here http://www.blood.co.uk/pages/platelets.html
It's rubbish - women can and do donate - I do ...... but there are few women as generally our platelet count is not high enough - we can also carry anitbodies which prevent donation - did you ever go and do it?