Hey OP, you're very right, there is definitely a gap in the market for a good co-ed in the Guildford market place. The boys certainly don't have the level of great choices afforded to the girls!
I find it really interesting how you mention Tormead as the likely candidate. Apart from GHS (which, as per United Learning's business model, and fundamentally their roaring top ten performance, will always remain girls only) there are a number of other conversion options out there. Such as St Cats and Priors Field.
I say this, because, unless there is a really strong business model, the "going co-ed" process usually comes about due to a fear of declining student numbers. The exception is usually when one is taking girls into a "boys only" school, in which case it is normally a combo of that plus a good way to raise grades.... which adding (smart) girls, sometimes stolen from girls' only schools, usually does.
Going co-ed for a girls school in Guildford, will, especially with RGS around sucking up the traditionally "smarter cohorts", cause grades to go down.
For a school like Tormead, which seems to attract solid numbers each year, and produce pretty decent results, for them what would be the point in going co-ed? A genuine question. Given that most girls' schools, tend to loose numbers from year 11 to sixth form, as our DDs see an attraction in going elsewhere to experience "life outside" / "boys" before Uni, If I was considering co-ed, it would be for sixth form only, with a rigorous entrance exam, to protect A-level performance.
I've seen the new Head speak, and despite his clear pedigree, co-ed experience and the way he communicates (which is pretty impressive, I have to say - and i'm a big critique of crappy public speaking!) it is clear that he does believe in the benefits of a single sex education.... At the very least he talks with perceived passion about one! So not sure if it would be forefront in his mind right now or not? I imagine, like most headteachers, he's trying to get everyone back into the swing of "normality"!
Going curve ball here...... But if i'm completely honest, I always thought that with the general appeal of boarding decreasing, leaving a school with a cost base built around one (higher than day only), that St.Cats would be the one to take boys first!
But then I freely admit that I'm quite off the wall, generally!
Good debate though OP!