For context - I'm building autonomous Escape Rooms that react to the player's actions. We currently have several long tracks that fit various sections of the games, and we switch between them at story junctures, by playing loud sound effects that mask the transitions.

I would pay in the order of thousands of $ (pounds in your case...) for this system running locally in my network, and in what I would call 'continuous mode'. By which I mean, I give it initial conditions and the track just evolves continuously forever[1].

Ideally, I could then send additional configuration messages and have it evolve further. e.g. Start with ambient/sparse. After a few minutes, I send a command to transition to ambient/sci-fi. Then later, to electronic/aggressive, with a seamless transition between. Even better if I can have a command that is essentially "react to this message", and it does something like a cymbal clash or whatever is appropriate for the settings.

I've done improvised theatre shows before with a jazz band that did all this for us, and it was AMAZING! Being able to do it with an automated system would be a game-changer for my business. I've been considering writing it myself, but it would cost at least tens of thousands of dollars worth of my time, and our current solution is sufficiently adequate that I'm not ready to do so yet.

[1] Ironically, given your current business model, I'd actually have almost zero interest in retaining the music after it's been played.)

On a positive note, I really enjoyed all the tracks I did make!