In a reply to a Freedom of Information request, Ofcom acknowledged they have never prosecuted a user of PMR 446

Ofcom were asked:

I was wondering if it's possible to give me figures as to how many prosecutions have been made against a person or persons for the specific offence relating to using equipment on the PMR 446 band (446Mhz) against the licencing conditions. I appreciate this is a licence-exempt band, but for example can you tell me if anyone has ever been prosecuted for using the equipment above the permitted .5 Watts or for using an external antenna.

Ofcom replied:

We are not aware of any prosecutions against any person(s) for specific offences relating to using equipment on the PMR 446 band (446Mhz) against the licencing conditions.

See Ofcom's full reply - https://www.ofcom.org.uk/__data/assets/pdf_file/0017/222731/Misuses-of-PMR446-Band-Prosecutions.pdf