Threat of Dismissal after accepting Voluntary

I have recently been offered VR by my employer and they have agreed and accepted my application.

However I am starting to think that my dropping off performance (lost all love for the job) and sickness absense may see me dismissed before the end date.

What I would like to know is;

Are my employer able/likely to dismiss me before the agreed leave date without comeback after i have effectively agreed to ending my own employment?

Any opinions help advice on how to play this would be gratefully received.

Are they able to - yes. Are they likely to - utterly no idea, my psychic powers being zilch! I can't see them being particularly keen to go to the hassle if you are leaving anyway, but that would depend on whether you push them too far. They won't if you don't give them a reason to.