Out of curiousity, how much is the difference?
The way i see it, you need to speak to someone, i would suggest check-in at Gatwick, purely because if its a handling agent, they are unlikely to know that its cheaper to fly the full distance. Just ask them, and if they say no, dont argue, just try...
...speaking to the captain (via the crew) sounds like a good bet. I'm sure if you are reasonably polite there wont be a problem. However, as I (and others) said, there could be a security issue. Certainly I might be concerned that a pax suddenly changing his plans at last minute may have left a device or something on board.
Getting off with the PLH passengers is not recommended. I am told WOW seats pax for the two destinations on different sides of the aisle anyway, so you will likely be noticed.
As a last resort, I flew with Eastern airways on their SOU-NCL-ABZ service, and was seated next to a very nervous passenger, who asked to be let off at NCL, rather than go through taking off and landing again to get to Aberdeen. It didnt seem to be a problem then, they simply asked her to identify her bag and she disembarked with all the other NCL pax... how goods your acting???
RTG!!!