If you want to try for constructive dismissal, get a report from your GP first, before you write the letter of resignation.

Make sure your colleagues know why you are resigning, and that copies of any resignation letters go to the bully's manager as well, so that they know exactly why you've been forced into this action. That way, the bully can't spread any 'bad vibes' about you and it may encourage other victims to speak up.

Good luck

LTS