> They even outsourced me to develop a ticketing system for a [potentially recognisable car service] company, to the tune of more than half my annual salary for four weeks work.
And it didn't occur to them that this meant you were worth more than they thought? They're hopeless. Run away. Take some of your vacation days and go interview someplace where you can be a profit center instead of a cost center.
You're a poster child for Stockholm Syndrome. You are completely and utterly playing by their rules. Just start looking for a new job, that is something you can do when not at work, and if things progress as far as you getting an interview, take the day off from your job and deal with that problem then. Call in "sick". Say you've got a funeral. Take a vacation day anyway. When you get there, you'll realize you don't actually care about your current workplace that much after all.
But by leaping to the problem first and then declaring that you can't solve it right now, you're setting yourself up for failure. You're just manufacturing excuses for staying in the position you're currently in. STOP. IT.
After he runs out of sick days, I'd advise him to just stop showing up for work on interview days. They're not going to fire him, he's not interested in staying there, and the bridge that he might conceivably burn is built out of bubble gum and toothpicks, anyway.
Half a day would be enough to interview at the nearby place that paid the equivalent of £102K/year for you. Ask for £80K/year, they'll think they're getting a bargain. If you save half of it, you'll still be much better off, and you'll be building up a reserve so that, later on, you can quit and spend time interviewing in a city where there are good jobs.
And it didn't occur to them that this meant you were worth more than they thought? They're hopeless. Run away. Take some of your vacation days and go interview someplace where you can be a profit center instead of a cost center.