If you have to go the private route, besides the prep departments for senior girls' schools Tormead, GHS and St Cat's (Bramley), other local preps to consider, depending upon your location, are Ripley Court (mixed), Cranmore (mixed), St Hilary's and Rydes Hill (mixed but boys leave after Year 2), Edgeborough, Hoebridge and Longacre (mixed) and Halstead (girls only).
However, I agree that occasional places do come up at good local state schools. We managed to get a reception place from the waiting list at a sought-after infant school within a term of starting at another well-regarded one (we moved to our closer school). There was also a lot of movement on the waiting list for a Yr3 place at Holy Trinity a few years ago. (I suspect people were sitting on places whilst weighing up whether to go private then or delay until Yr 7).
Most of Guildford's state schools are very good. At the secondary level, the recent Maths Challenge evening hosted by GHS saw Guildford County School achieve the highest results across the 4 years participating (Yrs 8-11), with 3 podium finishes and George Abbot were on the podium for at least one year group. I think some 10-12 local schools participated including the private 3 Guildford girls' schools, City of London Freemen's and Charterhouse. (RGS double-booked themselves.)
I also know of at least two County School sixth formers who've achieved Oxford offers in the last two years (if that kind of thing floats your boat, I appreciate that not everyone is that bothered by Oxbridge places and that other universities offer better courses for a variety of subjects). Good luck!