It's easily done, London life is crazy! There's always a new place to eat out, friends are calling to have a few after work drinks, there is temptation everywhere. The thing is, I don't think you should compromise your lifestyle just to be healthy. If you did that, I reckon it would last about as long as the standard fad diet, 2 weeks.
You are not alone! Everyone thinks being a trainer means you have all day to train and stay in shape, when in reality, we spend all day making other people fit, so finding time to work on our fitness is actually harder than you think! We all have the same issues; time management, and finding fun, varied ways to keep fit and stay in great shape.
(that's why you see so many trainers in the gym who need a trainer themselves)
Look, you can still enjoy your social life, and enjoy good food. It's about making sure you are good 80% of the time, and that if you are having a nice meal out, that you have worked out extra hard that day, or that you don't let the bad meal leak into the next day.
We work on this type of stuff every day with our clients, helping them overcome their weekly barriers, and making sure progress is being made constantly. Changing and tweaking things bit by bit.
I know how you feel. I totally get that your busy, you have a social life, and you want to enjoy your life. Fitness naturally takes a back seat, and before you know it, you have let your figure slip.