Being bullied by your manager is frightening and unpleasant, but I need to highlight that technically bullying is not illegal in UK. That does not mean that you should not take action.
If you have 2+years' employment service with the same employer, you need to formally lodge a grievance, in writing, outlining events, timescales, specifics of your manager's behaviours and that despite suffering ill health from these circumstances, you want to continue your employment with them, but to eliminate the detrimental effects of the stress caused, and to enable you to function in a safe and healthy environment, you wish to be considered for an immediate transfer to a different function. If you use those word, it could signal to them that you've consulted a solicitor and they are indirectly referring to the rights under the Health and Safety at Work Act 1974 which nowadays increasingly incorporates obligation by the employer towards safeguarding the employee on welfare matters such as the negative effects of bullying which have become alarmingly common, due to lack of action, probably because technically bullying isn't illegal per se
If you have x years' history of positive end of year appraisals, awards, positive feedback from internal staff or customers mention this in your grievance, to show tangible evidence of your performance. If the only person being negative towards you is this manager then it calls into question their bad character.
You need to have fully exhausted all internal routes to resolving this matter ie via your company's grievance + anti-bullying/harassment policy, before being able to instigate a Tribunal Claim. I would highlight here that Constructive Dismissal is a technically challenging and expensive claim to pursue with limited precedent of success, and in your current stressful condition, you would be advised to try to resolve through the formal grievance route rather than Tribunal, to get them to take you more seriously.
By you saying you didn't want to 'go formal' could have signalled you were not serious and they've chosen to bury their heads in the sand rather than deal with it. It's a very common response, ignore and hope it goes away.
If you have worked there less than 2 years, your rights are very restricted unless you can prove discrimination which doesn't sound relevant here. It would be better for your health to find a new job.