Can’t comment on regulations. But OCR is fairly standardised now and simple to build from scratch. I built my own as a hobby project a few years back. Hard to quantify how many actual hours of work it took, because I don’t keep track with personal projects, but I was done in a few weeks. But I’ve not had to put mine through the scrutiny of testing and bug fixing, which Monzo would no doubt need to do. Then of course there are the regulatory hurdles and timing, which I know nothing about.
I built mine purely because I could, and once I got to a point of oh cool it works, I’ve not touched it since. But it’s there now in case I ever need something like it for production, all I’d need to do is refine it and test test test.