No seatbelt in Tesco car park - enforcable?

[BigBazza]

Morning All,

My two nephews pulled into a local Tesco car park (to actually go shopping, not do donuts in a pink Saxo) and as they approached the space, they slipped their seat belts off.
There was a marked police car in the car park and the officers came over to the lads and did them for not wearing seat belts.
They explained they had just slipped them off ready for disembarkation but the officers said they didn;t believe them and gave them both a FP.
Are these FPs enforcable on what is essentially private land?

Thanks for any advice.

BB