Not putting a damper on the idea, but don't overlook that there may be insurance/liability issues in letting other people use home darkrooms, particularly (but not exclusively) if a charge is made.
Survey done, and this is one of the reasons I could not share my darkroom, apart from the fact it is very small and my own personal space.

However, I am sure Simon et al are fully aware of this - H&S in the UK is somewhat of a hot topic most of the time. What may however be a reasonable outcome is that a database of accessible darkrooms could appear, and there will be a few, that would otherwise remain unknown.