London is a great city if you have money to spend, time to spend it in, and you don't live too far away from where things are happening. But I would never have wanted to be a student there because:
- As a student you'll have very little money;
- As a medical student you'll have very little time; and
- A lot of the London Universities' accommodation is a long way from the hub of the university.
Contrast that with Sheffield. It is a cheap city to live in as a student; and the halls of residence are in the immediate vicinity of the university.
Honestly OP, everyone I know who went to Sheffield absolutely loved it. You've lucked out! I bet you come back to this thread in a year's time and tell us how happy you are.