Re: Ok but how much tax is fair?
"One answer is to abolish corporation tax altogether."
And also abolish multinationals, presumably? Otherwise US corporations would still be shovelling all their UK-made profits back home and not being subject to UK tax.
Country-by-country reporting is the correct way to go, and abolition of the multitude of loopholes through which you can transfer cash between countries. No IP licensing, market value for intra-company goods (e.g., Starbucks coffee beans should be sold to Starbucks UK at market price for beans), and so on.
Or you introduce expected tax: if the group as a whole made n% profit, and UK sales (as determined by where the customer is) are £m, then if the profit made by the UK arm falls significantly below £m * n%, then you have to open your books to the public, journalists, accountants, etc., who can decide if they believe you. If not, the profit assumed will be 3*m*n% and taxed accordingly.
It should help clean up accountants books as well, because if they are too complicated, you get the "I don't believe you" response.