My mate lives there and wants to hire some such bike to see if he likes the concept.
Any ideas? May not lead to a purchase, so he already suggested a demo would be taking the pee a bit. Unless you know of a really understanding bike shop.
There's Ubyk in Oxford. Not exactly local, though.
Singular is based not too far from HW. If your mate gets closer to buying and likes the look of the Swift or the Kite, it's worth contacting Sam to see if he's got anything demoable.
Swinley bike hub have a Gravel bike for hire. Book along, an hour or two on the forest tracks with a bit of single track will soon let you decide if it is what you want. If he already has an MTB he can combine it with a spin around the trails there
Saddle Safari are in Marlow, have been around a while and might be worth a call/visit.