Google currently says it has two datacentre locations in Europe - one in Belgium and one in Finland. Their advertising business was run for many years out of Eire, though I believe nowadays they do have a London office.

At any rate, if you are running Google Ads in the UK, your agreement is still with Google Ireland Limited, and if you aren't VAT registered, you pay them VAT at the Irish rate. (If you are VAT registered, you self-assess and pay VAT at the local rate)

So I'm guessing they pay most of their tax for UK business in Ireland?