River's gun looks like a repaint of the "toy" guns from Gridlock, that the hijackers used to kidnap Martha.
Gridlock:
River's gun:
Concept art of the Gridlock guns:
They're definitely inspired by the Nerf Nite-Finder, that's for sure. I can't determine if they're based off of one, or just a very close sculpt. For example, here's a modded nite-finder that's had the bottom bracket thing for holding the darts removed:
Compare that with River holding the gun and it looks like the underside of the trigger guard is spot-on. The overall shape of the handle is close, but the end cap, grip ridges and oval opening for the trigger are all different on the Gridlock/River gun. But they're definitely not the Maverick, which looks like this:
My running hypothesis has been that the BBC prop department got the concept art inspired by the Nite-Finder and either didn't realize it was based on a toy or couldn't get access to a real one, so they sculpted a whole new body from the art. It's the only way I can think to explain how close-but-no-cigar the handle and trigger are. Unless they realy bought a Nite-Finder then resculpted the battery cap, handgrip and trigger opening.