To be fair Elc if you are only holiday diving, the diving would probably be quite shallow so nitrogen narcosis would not be a problem and diving in warm waters is a long way removed from diving in the UK where the water is obviously much colder. So holiday diving may be OK
However if there is any risk of hypo whilst under water it could quickly end up as a serious incident which every dive school would want to avoid, so I can see their reticence. There is also the fact that diving does take it out of you, heavy equipment and time hanging around in tight fitting wet or dry suits can be quite exhausting, again increasing the risk of lowering blood sugars and hypo's.
You could try contacting the Professional Association of Diving Instructors (PADI) or the British Sub Aqua Club (BSAC) and asking for advice from them. Both organisations will have contact details that Google will find, it could be worth a call. Most holiday diving is organised by PADI dive schools as BSAC is more a club based organisation obviously UK based.
Hope this helps