Name comes from Royal Association for Disability and Rehabilitation (RADAR) key.
Extensive scheme in UK of disabled toilets in shopping centres, etc. Nice things about it are that the toilets are more likely to be clean, and also you don't have to ask somebody to come unlock it for you!
"A RADAR key, also known as an NKS key, is a blue and silver-coloured key that opens more than 10,000 disabled toilets across the UK. RADAR keys are used by some 400 local authorities to allow disabled people access to locked, accessible toilets.
The Royal Association for Disability and Rehabilitation, which is now Disability Rights UK, worked in partnership with Nicholls & Clarke, the inventors of the RADAR lock and together they created the National Key Scheme (NKS). The first RADAR locks were fitted in 1981 to help keep accessible toilets free and clean for disabled people.
Before RADAR locks were introduced, many establishments locked the accessible toilet themselves which meant that disabled people could only use the toilet on request. There were also cases where the key couldn't be located by the staff member, or the person who had it wasn't on duty that day. Fortunately, the introduction of the NKS meant disabled people could now use the toilet without having to ask someone if they could be let in."
There is something similar on mainland EU, a Euro key in the central states Germany to Slovakia - ish.
They missed an opportunity to call it a Eurinal key, I guess!
Alas the system in Ireland is much less extensive, and a different key again.