Congratulations to your DD mz!
Academics are important, and on that score I think you'll have to rule out Farnborough Hill in favour of Guildford High for a very bright child like yours unless you really want a Catholic education (and even then I don't think FH is particularly outstanding on that score either). Your DD's academic ability is such that it surely warrants the extra Â£3k a year that Guildford High will cost you.
Where Kendrick is concerned, the academics are much poorer than Guildford High's (and it's not just Oxbridge - Kendrick gets about 1/3 fewer of its students to the "second tier" universities too). But just as important are the other, non-academic, advantages that an education at a good independent school like Guildford High will offer. I've recently had to make a similar decision myself, and this was one major factor that swung things for me.
It's like the difference between a Picasso and a Picasso print - the original is worth every penny, if you can afford it