Hi pants46, good advice from Taz2 and clio and do not quit your job. Your employer has a duty of care and should be monitoring your absence, keeping in touch and eventially when you are up to it arranging for an Occupational Health assessment. I know you feel now you just want to give it all up but in a couple of months you may feel different and you've nothing to loose by waiting. If you feel better then you will beable to have a phased return with amended hours and duties on full pay. If you dont feel well enough to return it is up to your emoloyer to terminate your employment on the grounds that you are not capable to do the job through ill health, the time for this will depend on your length of service, as Taz2's time is a year, this is usually the maximum. Then you will beable to claim benefits.
I hope you feel better soon but please dont make any hasty decisions, wait till your feeling better then decide whats best for you.