CAP 773 and safetysense leaflet 25 give chapter and verse on GPS approaches and stress repeatedly that only published approaches can be used.
Even if there is a published approach to Pembrey - all GPS approaches are by definition Non Precision Approaches - how many that you know have a MDH of 200 or below?
I suspect the aircraft in question did have appropriate equipment and an instrument rated pilot so he should know better than to even consider an adhoc approach - especially when Cardiff was an available div with radar/ILS available and better weather. Fortunately for him he got a better weather report from his intended destination but, unless there were some big holes in the cloud, would have still had to perform an IMC descent to cloudbreak well below SAlt.