Take on board thete is a lottery element. It is far from first past the post. 60 girls, but take out boarders and your daughter will be competing for 40 places. These will be given to a, possibly predetermined, mix of scientists, mathematicians, linguists, classists, arts, humanities, social scientists, and mixes. Then you will have some capable of making real contibutions to school sport, drama, music etc. You dont know if you are up against two brilliant classists, say, or the best of a quite ordinary bunch, or competing against the future lead in the, very good, school drama.
Dd enjoyed the exams, because oftheir maths challenge approach. She enjoyed the interview. All along it was a case of nothing ventured, nothing gained. I suspect she may have got her place over brighter girls because of her strong extra curricular and potential contribution to school life.
Westminster is not for everyone. Changing schools and boys can be distracting. It runs on boy boarding school lines, and there is a lot of non acamedic stuff like Latin prayers and two full afternoons of sport a week. Dd had a wonderful time, and was really stretched in a broader sense, but this was not true of all her friends. If your aim is simply better grades Westminster will deliver a great education, but so too will a number of other London sixth forms.