The earliest time you can start to work without any other permission is the first day of your terminal leave. However, as long as you have your COs permission - best to have this in writing, you can work at any time during off duty hours. If you have basically been released from work by your unit there should be no problem with this at all. Just need to bear in mind if you are working full time that the army still has first call on your time until on terminal leave so if required you will be called in to camp - very rarely happens but you need to bear this in mind. Also you need to remember that tax and Nat ins is your responsibility, you should try to get your civvy employer to tax you using a BR code as your allowance is already deducted from your army pay, your Nat ins deductions will also, probably be wrong. Once discharged it wouldn't be a bad idea to get yourself a appointment with your local tax office to ensure you have paid the correct amounts, better than getting a bill later.
PW