My heart goes out to you, was in the same situation some years ago and was on sick leave for 9 mths while official complaint was processed. Returned to work but in different department - the difference was day and night. This was local government, no bullying policy in place at the time BUT 8 years later already damaged by my action was dismissed, no early pension etc.
My tips
Stay on sick leave, assume you have confided in GP the reason, if not do so
as you need to lay down evidence - may not be needed but is the start of taking some control. You could be sent by employer to independent doctor for assessment after a certain period, do not panic - GO explain the situation (you may get quite upset doing this, I did and had to look at the wall while trying to explain the work situation) strange I know but only way I could do it! Company doctor recommended I stay on sick leave.
Work on official complaint do give examples with details, dates and times - this can be tough/upsetting - I wrote it all down first just as it came emotionally and then was able to re-read it after a week or so to present a report.
Contact Union, ACAS, Labour Relations Agency (were very understanding, helpful and offered a session of free legal advice which was very empowering.
Maybe an employment lawyer if you can afford?
Be kind to yourself - enjoy the small things maybe a walk during the day, a few pot plants, yoga classes and breathing helped when I woke in panic in the early hours.
DO NOT allow yourself to feel guilt in any way for having to take sick leave because a sad, insecure rat/person is making themselves feel better by belittling you (and guessing others) - their problem not yours.
Look in the mirror and say aloud ' I will NOT allow NAME to do this to me or my life'
I felt such a fool doing this and only whispered at the start - it does help. Do this many times a day or when awake at night.
Take care of yourself