As I understand it 20% is wildly optimistic for the profitability of retail firms - it's an order of magnitude less. Services companies may be more profitable, but I doubt they hit 20% either.
Thing is, we already have a direct tax on the amount of sales a company makes (note - not revenue, which is slightly different). It's called VAT. Why not increase the percentage of VAT instead, rather than creating a new type of tax with new collection costs?
I've heard the arguments that VAT is a cost on the consumer rather than the businesses before btw - and it won't wash. Consumers foot the bill for everything every company does everywhere - always have, always will. Just because you want the "rich foreigners" to take the hit for your new tax plans doesn't mean it has the remotest chance of happening.
Whilst we're at it, why don't we lower business rates and corporation tax on all companies, and shift our expectations to society collecting their tithe from businesses via VAT instead? That'd be much more effective at levelling the playing field between large multinationals and small local businesses.