> As of 2017, the average annual real disposable income of Dutch households was roughly 31,500 euros.
Disposable household income would be, at a minimum, after tax, and possibly after accommodation. You'd want to increase that by about 60% to account for tax/etc, which would be 50.4K. That seems more reasonable to me, and would put the multiplier at 7.7.
Not sure about the UK - that could very well be accurate, but London throws averages completely out of whack.