Talking to my mum about BC

I really want to go on the pill as I am going to become sexually active. I would like to tell my mum as we are very close and she is very open about birth control and in the past has said when I am 16 if I needed any help getting any I should tell her and she wouldn't judge me for it. I am 15 now but would like to have been on the pill for a while before i become sexually active. Personally I am very embarrassed to tell her this (I find it easy to talk about it to my friends and boyfriend but awkward with my mum despite how understanding she is) how do I bring up the subject with her? Should I just go with a friend instead? (In the U.K. You can get free BC and they won't tell your parents even if you are underage) please help me come up with ideas on how to ask her!