Google found..
http://www.handles4doors.co.uk/advice/bathroom-lock-requirementsExternal Operation of Bathroom Locks
Quote:The most important regulation is that any bathroom lock, cloakroom lock or en-suite lock in any public building must be accessible from the outside; i.e., a bathroom lock must have the ability to be operated externally, in case of emergency. While this regulation doesn't necessarily extend to private residences, all our bathroom locks can be operated from the outside, for the sake of security and to protect you from any future amendments to the regulations.
The bathroom locks in public buildings must also be operable to those with limited mobility. While it's impractical to try to cater for every unique situation, it must at least be demonstrable that your bathroom locks have been designed to be comfortably operated by as many people as possible, regardless of personal circumstance. Again, this regulation doesn't apply to private homes but it's certainly worth considering anyway. We have a wide range of large, stylish, functional bathroom locks, any one of which would be finish your bathroom door off really well. However, if you're buying for a public building - and the Building Regulations define a public building as any building where people are employed; so your workplace is a public building - then you should first make sure that the lock is easily operable. A thumbturn lock, like the one you probably have in your bathroom at home, may not be adequate under the requirements.
Does social housing count as a public building or private residence?