There's a lot in this case that we don't know.
The lady may have been belligerent and annoyed when she phoned the police, to the extent that rather than risk a pointless argument, she was just fobbed off with "We'll look into it, madam." It may be that any attempt to point out the law, even one from the Chief Constable, would have been ignored "I'm a customer of yours, officer, and the customer's always right."
He comment on Facebook suggests that after speaking to the police she believed she was right and had won the argument, hence her posting

I'm all for naked hiking, but -2c?? Some real love for naturism right there! Not trying to stir things up, but just wondering if it's actually enjoyable with that weather, seems that those are the cases where clothes are indeed handy!

Yes yes yes. I am a regular walker where I live and love the tingling feeling I get all over my body when it is below zero. I either run to where I am going to walk naked or I cycle so I am pretty warm by the time I get there so it is really refreshing to take my clothes off...