The point about corporations is a valid one and one that has to be addressed at a government level. UKPLC have all sorts of levers that they could pull, which would garner an acceptable tax take but choose not to. All this bleating and whining from them about how companies need to "pay their fair share" is preposterous and basically amounts to asking companies to voluntarily structure their affairs to pay more tax than the law requires of them.
Take facebook as an example, say they have worldwide profit before tax of £1bn and 10% of their users are UK based (both totally fictitious numbers), what do you suppose would happen if the government said to facebook "give us £1bn x 10%(uk users) x 19% (UK corporation tax rate) or we'll block your access to UK citizens and the revenue they generate, because that's what we think is a fair amount of tax to pay"? Hardly an unpopular move politically and FB would soon cough up rather than lose UK revenue. Yes there is scope for tit-for-tat deductions by other countries, but ultimately you would end up at a point where if you "in reality" generate revenue in a country, you pay tax there. Hardly the worst situation in the world, and would put big companies on the same footing as your local corner shop. Maybe you have to assess big corps on a case by case basis,but HMRC already do that with the various wierd and wonderful schemes they come up with to avoid tax.
All private companies exist for the benefit of their shareholders and voluntarily paying more tax than they need to is in direct conflict with a company director's overriding directive to deliver shareholder value. Its the same with wages, company directors should, under their remit, pay as little as possible to effectively deliver shareholder value. Now minimum wage laws, the power of unions etc all play into what that minimum number looks like, but as a society we shouldn't be asking company directors to not do their jobs properly. It is the role of the government to legislate as to what companies can and cannot do.