All airlines, BA included, take the use of cellphones very seriously. However I think that you would have been better making a complaint through the proper channels, starting with the Cabin Services Director. The CSD on your flight did not receive any complaints against the crew. I don't thing that PPRuNe is the correct place to make anonimous accusations against clearly identifiable cabin crew members.
At that stage of the flight the cabin crew are usually very busy on a full flight (I'm very pleased to hear that it was full), so it would be unusual to have the time to make a 7 minute call. You also state that the aircraft was fighting heavy winds at 7000ft. At 7000ft the aircraft was over BNN VOR, heading SE with a slight tailwind.
I'm sure that if you make an official complaint, it will be fully investigated, as her mobile phone records will prove or disprove the accusation. However, as you are making a serious allegation, you will leave yourself open to legal action if she can prove her innocence.
Airclues
ps. I hope that you were more successful on your connecting flight to Munich.