You don't understand how VAT works.
If I spend £100 on AdWords as a company, the £20 VAT comes off my VAT bill. I pay £20 less.
So Google's VAT bill is utterly inconsequential and meaningless.
VAT is only paid once by consumers for products, advertising is a company expense that simply shifts the VAT one level down the chain. It doesn't get applied at every level on the supply chain, it gets passed down.
So if Amy's Amazing Tours sells a £100000 tour round London, without any advertising, it generates £20000 VAT paid by Amy.
If the same tour is sold by Mike, for the same price of a 100k, but with £50000 worth of Google adverts, Mike pays only £10000 VAT with Google paying £10000 too. Still adds up to £20000.
Fact is, Google are using the UK's infrastructure, our commons, our laws, our police force, our army, our NHS, our roads, our telecommunications, etc. to do business. And need to contribute to it. Plus we tax economic activity to support our welfare systems for our society and Google/Facebook/etc. are circumventing that.