1 Even if you work no overtime at all, you should still earn £40K+ (including booked Sundays - which are like guaranteed overtime - night shift premium and Inner London allowance).
2 Depends on how your route interprets the HR rules. The default used to be that you would be expected to work a minimum of a year at your first box before moving on, but it wasn't (isn't?) always enforced. My first route did; others don't. More recently, when all new starters were classed as apprentices, I believe they had to work at least a year from the date they started signalling school before being able to move. It's a question you can ask at an interview.
3 What do you mean by the "transfer process"? You'd apply for suitable jobs advertised on the internal job list in the lead-up to your minimum service period ending (whether a year, or something else) and go from there. The main delay often occurs after you've accepted a new job - your current LOM (manager) may only want to release you to your new role once they have a replacement ready to step in. You can often find yourself part of a chain, where you're stuck until your replacement arrives, your replacement is stuck until their replacement arrives, etc etc.