Yes, they call the police and an additional fine can be added
"Call the police for assistance in the following cases:
the offender refuses to give you their details
you suspect the offender has given you false details
A police community support officer (PCSO) can detain the offender for up to 30 minutes before a police constable arrives.
The offender can be fined an additional amount on top of the FPN, if they refuse to give you details, or provide false details."
https://www.gov.uk/guidance/enforcement-officers-issuing-fixed-penalty-notices