While constructive dismissal is definitely an option, and advice above is good, ESPECIALLY detailing the reasons in the letter if you do resign (including stating that you feel you have no option but to resign), be aware that CD is one of the harder ones to prove (and win) at tribunal. You will struggle to get a lawyer to take it on pro bono, added to which you have to front up some cash these days to even go to ET.
That said, some employers will take a risk based approach and offer some sort of payout (with a “compromise agreement” including confidentiality commitments from both sides) if there is even a threat of a tribunal.
Also, if you are in one of those industries where everyone knows everyone (no idea if this applies), the reality is that a potential employee who has a history of taking people to ET can be a barrier to being hired. Shouldn’t be, but in reality, it can.
Definitely look to get out one way or another if this job is affecting your health (including mental) which it really sounds like it is.