This environment is noxious to your mental health, your sense of self-worth, your ability to get decent work in the future, and so on.
Quit. Now.
After you've handed in your notice, and they're exploding into fireworks of anger and "I can't believe this shit"-ness, tell them politely that you're available for consulting work with them over the weekends if they need help with the systems you've built for them, at a reasonable rate of £200/day.
Then go look for another job. You'll find one. IT staff are always in demand. If there really are no jobs where you live, move.
I don't necessarily disagree, but it can harder to find a new job if you don't already have one. You also have to explain to your new employer why you left your previous job, and "the place was a nightmare," even if true, is rarely a good thing to say. Your new employer may wonder if you are dependable.
Whatever you do, do not accept a counter-offer, and do not let your boss guilt-trip you into staying. When you decide to leave, you are gone.
The job search can take a long time, so don't get discouraged if you don't find something immediately -- prepare yourself for the fact that it may take a few months. If it happens any sooner, then you will be pleasantly surprised.
This environment is noxious to your mental health, your sense of self-worth, your ability to get decent work in the future, and so on.
Quit. Now.
After you've handed in your notice, and they're exploding into fireworks of anger and "I can't believe this shit"-ness, tell them politely that you're available for consulting work with them over the weekends if they need help with the systems you've built for them, at a reasonable rate of £200/day.
Then go look for another job. You'll find one. IT staff are always in demand. If there really are no jobs where you live, move.