If you have a disability and particularly if you use a wheelchair normal public toilets are hard to use being space constrained and not usually accessible. Disabled toilets are usually available on large public premises, but frequently locked to restrict access. The Radar National Key system aims to make all disabled access toilets use the same lock and at the moment have reached over 400 local authorities and 9000 individual sites across the UK.
Keys can be purchased from Disability Rights UK shops and some local authorities, but pirated copies are widely available online. The keys have a distinctive long stalk and large head both to make it more difficult to lose and to provide extra leverage for those with reduced grip. This does unfortunately make them difficult to add to a normal set of keys, but they fit well in a bag or attach them to your wheelchair.
If you have reduced mobility you should have a Radar key. They are inexpensive and priceless at the right moment.