London it is one of the most beautiful cities in the world but to experience it to the fullest before leaving it is good to know 3 essential things which allowed me to visit the city well without too many hitches. Among the first things, never, never, never forget the adapter for the power socket: D
Remember the time zone, when you arrive in London in fact you will be an hour behind Italy and if you don't have an iPhone or phones that update themselves you have to do it yourself and I recommend that in a trip the times are very important: subway, train, museum hours and many other things.
Don't forget to change the currency before leaving. About this experience I I have already said mine in another post and in my opinion it is better to change a few pounds first. But remember that if you have to go to the post office there they don't have much cash available, maybe take the minimum and try to pay with the ATM once in London, there they really use the ATM everywhere.
About theadapter as I mentioned before, try not to forget it. There are also many international adapters and by buying one of those you can use it in all parts of the world, even in London. If you go to a hotel and you have forgotten it, they could lend it to you, although unfortunately they sometimes charge for it. If, on the other hand, you have taken an apartment you will have no choice and will have to go and buy it. Always better to take it from home;)