Interesting. I still have crutches from 2005 (NHS-issued) when I broke an ankle. Nobody requested that I return them but I did try to do so, and whoever it was that I saw quite politely and convincingly explained that the paperwork involved would actually cost the NHS more than the value of the crutches. This made sense to me at the time and makes even more sense now (I work for a large corporation, private sector, and the bureaucracy can seem inefficient in terms of man hours – I am told that in the public sector it is far worse)
I am surprised that you were asked to return yours due to their value. Were they fancy? Mine were the standard aluminum tubing ones, I imagine they cost the NHS no more than £20 wholesale.
They are handy for getting free priority boarding on cheap flights (I have NEVER done this) 😉