Hello,
My reasons not to drink are different but the excuses are equally valid. I found difficult to keep up with the level of drinking when I arrived to the UK, but I never said "no" to a drink as people did not feel comfortable around me and saw me as kind of antisocial. Every time I had had enough, or I just did not feel like drinking, I would get my lips to it and then "loose" it somewhere in the bar, or I would go to the toilet with it and come back with lots less every time, I would swap it with my husband's (then boyfriend) empty one, and anything I could think of to get rid of of it. The funniest thing is people used to comment on HOW DRUNK I was this or that night!!
If you are at a dinner party with wine on the table and people serving it, you can always "warn" people beforehand that you have an ENORMOUS hangover and that one more drop of alcohol would push you over the edge...
Other excuses are detoxing for a week (so you tell people you are all right as it is only one weekend of "suffering") or that you have had a routine blood test that has showed really high levels of triglycerides and the doctor has said NO NO NO to alcohol, seafood, saturated fats, and refined sugars (white sugar), at least until the next blood test, to see if it is a "self inflicted" and momentary problem or a health issue...
Good luck!