Ex-HCC LD 1 252 FCG.

The L/D (252 FCG) was an ex-Hampshire County Council Ambulance used by Thames TV. This actual photo was taken during filming of 'The Professionals' 1977-83 (London Weekend Television LWT), but the vehicle appeared in a variety of other programmes much earlier than this.
Although some later Hampshire L/D's would have looked similar, this particular vehicle would not have had a reflective waistband whilst in-service.
Also, I note that the old acorn beacon has also been replaced with a more modern version. The vehicle was originally cream in colour, however, it was re-sprayed white (possibly the same time as the stripe was added), which I presume was designed to extend it's filming career.
Thanks to Ron Lilly for the photo and information.