EDIT: People keep saying that it's fine if they're obeying the law. There's a chance that they might not be obeying the law. While they're not evading tax (not using clearly illegal means) they're not just tax planning. Indeed, the schemes used by these companies are pushing a language change. Tax avoidance used to mean normal tax planning.
The tactics of Google, Amazon, Vodafone, Starbucks, etc etc etc mean that "Tax avoidance" now means complex schemes which are probably legal, but that might not be. The schemes are complex, and require believing odd things to be true. For example, Starbucks makes no profit in the UK[1], something that people will find baffling if they've ever bought a £4 coffee.
Any one of the big four accountancy firms has more staff that the HMRC. There's big money to be made in avoiding tax. When firms are caught they negotiate deals to repay some, but not nearly all tax.[2]
But the final slap in the face is that underfunded, understaffed HMRC gets "help" from the big four accountancy firms. What's the catch? What could possibly go wrong with allowing the poacher to play at gamekeeper? They lend staff to HMRC to draft tax law; they then use their knowledge of those laws (that they helped to create) to exploit loopholes.[3]
People really don't mind if you plan your taxes to reduce the bill. But they do get angry when they see people "taking the piss", especially when the country is going through austerity measures and when the NHS is being crunched. All these companies make use of UK infrastructure (police; education; health; roads, etc etc) - they should pay their tax.
[1] (http://www.independent.co.uk/news/uk/home-news/starbucks-is-...)
[2] (http://www.guardian.co.uk/politics/2011/dec/20/inland-revenu...)
[3a] (http://www.ion.icaew.com/TaxFaculty/26745)
[3b] (http://www.guardian.co.uk/business/2013/apr/26/accountancy-f...)