My guess is that it's worth a lot!
How much depends on whether the parts are originals or re-pops. Same for paint, though the BMX guys are a lot less bothered it seems about paying for a new finish if it needs it to be minty. And you've had it 15 years but how old is it?
I'd suggest having a very thorough look around on Rad BMX forum
http://www.radbmx.co.uk/ and looking at prices of similar bikes in the for sale section. Be aware that folk like a bargain and if someone unscrupulous thinks you don't know its true value they may try to take advantage of that. So do your homework first. It would probably also help if you had an idea of how much money you would like for it. It doesn't matter if it's worth £xx if you wouldn't sell it for less than £xxx!
But if it were me, unless you need the money or the space I'd keep it.
That said, the arse has to fall out of the BMX market soon, so you might get top dollar selling sooner rather than later.