I feel for you.
I made a similar leap from an OK-paying regional job at an established niche cashcow to a London start-up. The perks: a coffee machine, senior pay cheque.
The reality: a shit coffee machine, low moral, no realistic prospect of a profit and no willingness to pivot. 80% of costs have been sunk on a Facebook clone 'with a spin' while the owners try to sell snake oil to investors. We operate like the bad slides of 'Good vs Bad Startup' are a blueprint for success. The owners, almost weekly, come up with an idea someone has to coach them away from - a long ironically drawn out confrontational meeting of: "we don't have the resources". The bit I expected to pay off (medical monitoring hardware) turn out to be crappy I2C/SPI bridges any e.eng graduate could whip out in a month :(
I've had long look at myself: how could I turn down other offers yet accept this?! I'm still unsure why. For a while I feared that I deserved this - I'm one of them, one of the guys who decided to create their own header based HTTP authentication system key by a timestamp: a crap programmer.
It's been 4 months. Poor tests are still committed despite my best efforts to teach the one doing it that they should test a result not implementation (only 1 test damn it!). We don't use any JS frameworks on the front-end (yet alone my other true love: Typescript) because the last lead dev couldn't understand the immeasurable benefits of model binding. I've done - by no means single handedly - an incremental rewrite of the entire code base (front, middle and back). Although they're paying me on time, they missed my post probation increase :( I could go on but this is probably not the right forum.
In short: it's not worked out for me either. Thankfully I've got an excellent track record so I'm off in 3 days to a proper PLC. I heard large companies, like the BBC have 15 (FIFTEEN) designers in their News division alone... I can't wait to have just one designer unencumbered, available and talented.
The whole saga has left me with the strong suspicion most startups are a joke (no I don't want to be a DBA/CSS/JS and C# guru who maintains the iOS codebase!).
Inept owners who're unwilling to pivot, often trapped by the sunk cost fallacy, or owners expect to create a market with their 'one true solution [to a problem you no-one has]' plague the startup scene. No product should ever be a Facebook clone :(
For what it's worth, if I was hiring now I'd interview you solely on your written language skills (which are better than most native English speakers, myself included).