cut a long story short, they got quite UPSET that I don't want to drink
Some people get very defensive when you're dieting/making some sort of lifestyle change because:-
1) they're so entrenched in their habits they simply can't comprehend life without them;
2) Your abstemiousness highlights their lack thereof;
3) it makes people feel guilty, self-conscious or even 'lacking' that you're making a positive choice and they're not.
4) They are accustomed to you as you are, and may not want you to change, because then you may be different, or better.
(Have you not noticed a certain amount of resistance on a certain other forum? It's why I've not said owt. CBA with the criticism. Most of that lot are as supportive as Primark undies.)
Therefore they often attempt, even unconsciously, to sabotage your attempts to change to preserve the status quo.
None of the above are your fault at all, but may be aided by a calm sit-down explanation that this is important to you, and that you are trying to make a change for the better (even though you will remain the same person) and you would like them to respect that. Not a row, not to get defensive yourself, but a calm but firm chat.
If push comes to shove, blame it on your back problem/being off dairy/on tablets incompatible with alcohol... 'I have to do this cos [insert excuse here].... it's not like I really want to, but you know what doctors are like
'