Instead of walking out, which may seem like a solution, but will only cause you more stress in the long run, why don't you go off sick? For the first week, it's self-cert anyway, and then you can go to your GP to get it prolonged if you are not ready to go back.
I know you're scared of being signed off, but being unemployed with bills to pay is far worse, so use it to take a breather and assess the situation.
If you leave with having taken more holiday than what your accrued entitlement is, then yes they will deduct it from your final salary. When you say "large sums", surely it can't be that much?
If they are causing you to leave through their actions, you
may have a case for constructive unfair dismissal, but it will depend on a lot of different factors, and constructive, because of its insidious nature, is a lot harder to prove than straightforward unfair, which doesn't mean it can't be done, far from it!
I would strongly advise you to take the time off sick rather than just walk out, when you are under that much pressure, it is hard to think straight,so give yourself the time to clear the cobwebs.

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