If you go to
Welcome to the UK Civil Aviation Authority and go to medical section all will be explained
The class 1 medical reverts to a class 2 medical once the year is up it even has a section with a date on it for class 2 validilty date.
Then when you need the class one valid for getting the CPL issued you just go to any AME 1 and they will renew it.
The advantage of getting it ASAP is that the intail requirements are higher than the renewal. The price of it is only going to go up. And if they do find an issue you haven;t just spent the last year and loads of money flight training when you going to have to find something else to do anyway.