In the US I think it depends on airport and airline can't say for sure about the UK or India. The airports I was at allowed it but the airplane did not and neither did the Norwegian cruise line that went to 3 different islands.
Not sure what your gear is but I left most of my juice in stow-away and took one ego and 1 30 ml for carry on. I also had about 30 mini's fling from white cloud. Try 1 or 2 before your flight but you can rap that thing in your fist and just have 2-3 mm sticking out the top of your hand. As long you don't accidentally cough I don't think a single person even noticed.
Side note I was on window seat and had wife next to me. If you don't know your direct passenger next to they might wonder why you keep sucking a white straw thing while holding your fist supper close to your mouth

. So do it on airplane at own risk...and think you would be completely fine in lavatory. I would still hold it in and don't do any medium/large clouds. Exhale into your elbow sleeve as well.
I did that in airport as well as I like myself really didn't want to draw attention to myself. Either that or bathroom.
I'm pretty sure it's illegal to have any cameras in the lavatory or any bathroom...I'd be really surprised if it was allowed in the UK or India. At any rate check your local laws but if want to you can hide it really well with a mini fling as long as you practice holding it discreetly in fist.
Fair warning they were super cheap when I bought them during a November special. Around $1.25 a piece I believe. I think they are back around $3.00 which I really couldn't justify for day to day use or a long trip.
On cruise I used mostly standard ego and was just discrete about it. The did allow smoking in the casino so I got my vamo out a few nights and spent some time in there.