Tani, that's my drink of choice too! I have slim line tonic, it tastes just like the ordinary one, and saves a handful of calories and makes me feel virtuous.
I can take or leave alcohol, but it's nice once in a while.
I'd add a caveat saying it's best to eat before you go to the pub. If I have a couple of gin and tonics on an empty stomach I am nibbling on crisps and nuts, and that just compounds the old calorie problem.