You can develop the accent over there, dont go over with a fake accent you tried to create, they will just laugh. Just adapt to the language, trust me you wont even notice you do.
Make friends like you would here, just be nice, cool, calm and collected. Dont be a try hard trying to fit in and be a bitch, be yourself and be nice. Other kids from london will associate with each other like any other would, they would have some differences but thats the same for every school and anywhere you go.
I suggest taking both, you'll have a locker, you can change your bag. Im not from london so i cant help you with the bag situation but seriously no ones going to be like 'she has the wrong bag, kill her'. You'll be fine, they are just as friendly as any other human.
There is a change room in every school, its mandatory for changing for PDHPE, if there is no changeroom there has to be a bathroom. It is against national standards and the border of studies to allow the opposite gender to change in the same room. They will look after you, and everyone will know your new so they will all help you.
Just remember if this is the most difficult time in your life, you're laughing. Its only a few hours of your life a day, Youll be fine. Good luck :)