Well, we didn't quite see the far end, but seeing as we've seen the three ventilation holes, I thought it would be worth actually seeing some of the tunnel! This is the Clifton end, adjacent Clifton Down Station.
The tunnel is dead straight. You can, apparently, see right from one end to the other, though I don't think there's anywhere for a member of the public to stand that allows that view. Of course, in the days of steam, the view was mostly obscured by smoke from the trains themselves, and this exit had a succession of jury-rigged bell systems so that the driver would know when to apply the brakes to avoid overshooting the station, as it was hard to tell when they were about to emerge from the tunnel.
GPS Coordinates:
51.46433, -2.61331
Location: Whatley Road