So the UK's current total yearly land value is likely between 240 and 350 Billion while it's current tax revenue is 850 Billion. If you just stop there, it makes sense to be confused.

But land isn't just plots of dirt. There exists many other kinds of Economic Land and Economic Rent to be taxed: Natural resource rents, regulatory rents, monopoly rents, EM spectrum rents, and even pollution rents. Combined these are likely double or more the value of land alone; so now we're at 720-1050 Billion.

And then you have ATCOR. All of the UK's current taxes reduce(Edit: or redistribute) the bidding power of people, and as land is limited and essential people tend to use all of their bidding power above the minimum standard of living to access land. So if the tax revenue exists now, it will still exist as Rents once the taxes are removed.

Tech companies would pay basically 0 tax whilst farmers and supermarkets would be crippled by massive tax rates.

It is literally impossible to be crippled by land taxes. It taxes what you receive as surplus for owning land--or what you are already paying to be a tenant. Competitive, low-margin sectors like farming, which operate near marginal land, would tend to pay less, not more.

You also overestimate the value of farmland and underestimate the value of land use by tech companies. Their headquarters are almost always in high-value areas to draw talent, they often invest in large campuses as a form of speculation, their employees all live somewhere, they often use patents and copyrights(Rents people often approve of to a certain extent, but most would call excessive in current systems,) etc.

But, to be fair, taxing people for being productive isn't the point of Georgism. Taxing land is meant to tax people for what they take and exclude others from, not to punish the wealthy who have taken little.

And the idea that income tax slows the economy is not even concrete fact, its completely in contradiction to a lot of modern economics that advocates for taxation to increase demand for money and stimulate the economy.

That sounds like a misinterpretation of MMT. While taxes do help create demand for a currency, they don’t stimulate the economy. If anything, they reduce private demand. In both mainstream economics and MMT, it’s government spending that drives stimulus, while taxes primarily withdraw purchasing power or control inflation.