> Would you levy UK taxes on every penny Google makes, simply because their sites are accessible in the UK and used by UK residents? That would make no sense.

You check they're not using weird accounting to set up a (tiny) business in a low tax regime, and then syphoning money out of the high tax regime into that low tax regime.

Google has customers in the UK buying ads, and people in the UK viewing ads - it seems reasonable that they should pay UK tax on those bits.