It really is not a good idea to drink in uniform and it is certainly not allowed in my company or any that I have worked for.
I have though seen many crew from a major European National Flag Carrier check into a hotel for a nightstop and go straight to the bar where they drink in uniform in front of everyone. The hotel in question was my home for 5 months last year and there were always around 24 of our crew there who would always change into 'civvies' before entering the bar. One particular Captain of said national carrier was drinking until around 01.00 on one occasion and I boarded his flight as a pax 7 hours later. Not good!!!
With regard to positioning in uniform, I usually do this to make the journey smoother. I often have to catch 3 flights to get to work and being in uniform means I can go through the 'crew channel' at busy hubs which is handy when the connection is a little tight.
It also means that I arrive at my workplace with my uniform (and spare undies, shirt etc in hand luggage) as my main articles of luggage often arrive a day or two after me!!!
I hope this answers your question.
aero