I think the fact it was made in 1991 would be a big clue. No ambulance service is likely to have a vehicle that old in regular service.
Whilst no NHS ambulance service is likely to, the industry is now awash with private ambulance providers, that often just involve a self employed bloke with an old ambulance, a uniform and a glorified first aid certificate, who provide first aid cover at public events in return for a substantail fee. Most NHS ambulance services actually dispose of their fleet to such private providers, so I doubt anyone would bat an eyelid at a 20+ year old marked up ambulance these days.
As for keeping the blue lights and ambulance markings, something else you might want to consider are the practicalities of driving around in a marked up ambulance. Sadly there is a common misconception amongst the general public that everyone who drives an ambulance is a Paramedic. Should you come across an accident, or incident whilst driving this ex ambulance, you'll probably get approached/flagged down to help. In the heat of the moment a distressed/panicking member of the public isn't likely to notice that you're just some guy driving a converted old ambulance, and when they discover that you aren't actually a medical professional, they may well turn nasty. I've known this happen to a few non medical staff over the years (mechanics, admin staff, cleaners etc.) who happen to be moving an off duty ambulance from A to B and they've found it to be quite an unpleasant experience, I've even known some who now refuse to do it as a result. In the case of using it as a Campervan, also be prepared for a constant flow of people knocking on your door at every campsite you stay on, because little johnny has hurt himself, their wife has gone into premature labour, or their Grandad is having an angina attack and they want you to come and help!
