The only issue I would have is with the speadsheet, if this could somehow be kept in a post online that would be fantastic.
I have to do spreadsheet-to-web conversions fairly often with my job. If the Chancellor-in-residence wants to send me whatever spreadsheet they have, I could make it web-accessible with just a little bit of work, regardless of the format.

I do like the autonomy of this setup a lot. At first I was concerned about the fixed (rather than percentage) tax rates, but that does make sense. I guess with the test run we can see which tax levels work and which don't.

Saying of which, with the test run are we playing nicely and not trying to break the system? Or are we testing the limits?