Deepmind have at least a few thousand employees, I'm willing to go out on a limb and say that the industry doesn't have "hundreds of thousands" of people working on protein folding alone.
What's important is I think which people they hired, as well as Deepmind-internal infrastructures. You can just walk over to one of the world experts in deep learning and ask them how to address some issue.
In pharma companies they have people who are really smart about drugs and biology, but ML experts would all have to be hired externally, and likely don't represent the top of the field as well as Deepmind does.
I don't know how UK R&D tax works, but most countries allow companies to write off some profits against R&D. Apart from the fact Google wants to do research, having a large center based in the UK probably helps their UK (and formerly EU?) tax position.