Facilities rental is great, and £500k is not to be sniffed at, but if the facilities are taking a £500k hit, in terms of use, those facilities are going to get tired out pretty damn quick, and with no money left to keep things tip-top, just cash being used to 'survive since vat', then I'd be worried the longevity of the facilities and their availability to students is being compromised by a nessesity to rent them out to keep going.
They, themselves, would be wise to T-ing up with a larger school or group, who can provide some stability and see them right again.
There are many success stories of this. It's not quite the same scale, but Rydes Hill, another lovely school (like Priors Field) was on the edge of collapse, Tormead saved it and invested, and now it's rammed. In fact it likely has the most popular nursery in the guldford area and must be making Tormead a pretty penny, whilst giving them a co-ed option off the main site.
Priors field need to think outside the box now, approach a bigger school, and get a link. Like Cranleigh (just droped £10k off their fees) or StCats, they're in the middle of nowhere and will naturally struggle for catchment. These days, no matter how many buses you throw at it, kids want to be near friends, home, public transport and civilisation... but, at least stCats and Cranleigh have some academics, sport and cash to fall back on, priors field is a struggle.
I just hope they don't try and do this alone, they've been around for so long, and deserve to carry on, but Royal School Mark 2 is closing in.