Sometimes this just happens. I've had a car sitting in my garage since I stopped driving it in 1996. I'd owned it since new, but by then it would have been worth just a few hundred pounds, despite being in nearly-new condition. So I just parked it up in a corner of my garage, where it's been sitting ever since. And it has a private plate with my initials on it. One day I intend to get it back on the road (along with the other two it's sharing the garage with), but if that fails to materialise, they could well become someone else's 'barn find'...^^Reported.
Re: the OP - it's a shame to see it sitting for so long, must be a story to it. The plate has some value but would require the car to be MOT'd etc in order to transfer - I suspect after 17 years the costs would be greater than the plate's worth