This is a tough one to answer - not that we don't want to help - but the pricing of airheads, and very much so the pricing of R65 models, varies tremendously by region. And time of year. In the USA, an R65LS in condition like your with panniers could be worth $3K or a bit more, in my area. Out in California, especially near SanFransisco for some reason, I've seen them sell easily for $4000 or a bit more. Some places in the south and midwest part of the US, even a nice clean bike may struggle to get $2500. Selling in the winter time over here, when only the folks in the south get much riding in, causes prices to tank.
As I don't know the specifics of your area or the airhead market in the UK, I think only some of our members from over your way are likely to be of much help.
I hope you are able to solve your cash problem without having to sell the bike - yes, it is only a motorcycle. But it is the sort of bird that doesn't often come around to one's garage.