I think a really nice mint one would be about £300 to the right person, most of them would be £150-£200.
That one is corroded and will need a re-blue plus the internals looking at. They ran 'O' rings on the piston so at least you won't have any plastic cheese to deal with.
I think that would would be about £80-120 as it is, depends who is desperate for one. An interesting rifle from the 'airgun renaissance' of the late 70s. A copy of a Hammerli sidelever, made larger for more power, fitted into a military-style stock for Walts and the younger crowd, using the short barrel that Cardew said all a springer needed, fitted with the red-dot sight which the advertising implied was a laser designator. AGW had the tester go after clay pigeons with it ... One of the first full-power rifles completely designed by a marketing impulse. More expensive than an HW35 and not as good a performer, which is why the 35 is still in production. The Sussex Armoury Jackals lead to NSP Engineering and then on to Air Arms, so the AR7 has a small but significant place in British airgun history.