Hi Stu,
These are quite common for Flat Entrance Doors but in practice are often locked when the resisdent is at home to avoid an uninvited visitor. From a fire resistance perspective, only doors with evidence of fire performance in an unlatched configuration are suitable. Flat Entrance Doors of course are also held shut with a self-closing device.
My understanding in terms of CPR, is that such night latches are outside the scope of CE & UKCA marking. https://www.legislation.gov.uk/uksi/2013/1387/contents
However, many fire door manufacturers require that locks fitted to their fire doors are CE or UKCA marked. Therefore, product performance certificate would, in these cases, be invalid.