You can force markets to make an open API funded by participants, but you can't make it fully open (open API and open participation) if you don't want tax funding because then there's no funding source left. Maybe you decide a fully open market is worth being tax funded, though it's still mostly going to benefit very rich people.

Why is "no tax funding" a hard requirement? I would think the lowest cost to get that market information publicly shared and forthcoming (and thus increasing the efficiency and effectiveness of the market), would be for a government dept to operate the frontend distribution, but require regulatory submissions of transaction data within reasonable latency windows. It's just a modern form of say US SEC document submissions and EDGAR to distribute.

I don't see why you can't do that?

There's plenty of things that cost money that legislation forces companies to do anyway, regardless of if there is a 'funding source' or not.

Although this particular one is from the FCA so presumably is taxpayer funded anyway.