Patchway, not very far away, has lifts, is a GWR station and I'm fairly sure it's unstaffed.
Is it purely penny pinching the UK doesn't go for more glass lifts, or are there concerns about the machinery being exposed?
Given the proximity of Ashley Down to Bristol Temple Meads, should the lift not be available a passenger could relatively easily be carried on to BTM (or even Stapleton Road) to circulate back to the Up platform or get a taxi. If a lift breaks down unexpectedly there are presumably engineers based in Bristol that can be called out relatively quickly.