Is this compliant with BS 3621?

Unfortunately, this lock doesn’t comply to BS3621, but it does comply to BS EN 12209

Euro profile cylinder horizontal mortice lock for use on external wooden hinged doors providing a high level of security. CE Marked door lock Fire rated to BS EN 1634-1:2000 & Performance rated to BS EN 12209. Latch bolt operated by knob from either side Deadbolt operated by key from either side using a double cylinder or by key outside and thumb turn inside using a single cylinder with turn.