the doc can still give you a note even if you wanted to seklf cert, noone can force you into work if you are unfit, especially if you are only just taking the sick leave. they cannot demand anything of you, it os bullying and intimidating and they are committing an offence to speak to you that way.
you should contact the authorising body for the LEV thing and explain they should be able to accomodate you. i hate firms like this, they are very unprofessional in their approach to staff