Difficult to support but an interesting claim all the same:
Johnson wants to retain Scotland precisely because he knows London needs its economic support. Ignore the rigged data in Government Expenditure and Revenue Scotland. The reality is that little England knows that it is sucking value from Scotland as hard as it can. And it does not want that to stop. And the chance that it might is what petrifies them.
Tax rates, allowances and the progressivity of the tax system are the same across the larger nation. Incomes in London are substantially higher than elsewhere. Quite how Londoners are paying less in that others, elsewhere, with lower incomes and tax bills is difficult to work out.