Starbucks didn't do themselves any favours in front of the Commons (UK's most important parliamentary chamber) select committee investigating low levels of Corporation Tax. Like most taxes, CT is avoidable if you know how. Most large multinationals with multiple overseas operations attempt to minimise their liability in individual countries (a) to simplify the recovery of tax through the patchy double tax agreements that exist and (b) to ensure, as far as possible that their investors do not suffer tax twice on any given income stream.
The system varies from country to country but in essence any company is liable in their home economy for tax on their earnings. That tax will form the basis of a credit against the shareholders' liability for tax usually at a basic rate. The shareholder only becomes liable for tax on their dividends in excess of that basic rate. So, in the UK, a higher rate tax payer would only pay tax on their dividends at the difference between the basic rate and the prevailing higher rate of tax. In effect, corporation tax is a payment of tax on an earnings stream in advance for the shareholder. So, the tax gets paid somewhere.
If a US company, operating in the UK, pays CT here it can claim that CT payment against its US liability through the prevailing double tax agreement. The problem with this system is that Starbucks, trades in a very large number of countries. To avoid the problem of having to reclaim under the double tax rules in every country it tries to minimise its liability in each country but, make no mistake, ultimately the tax will be paid.
So why, giving that proportionately the UK benefits from the way the system works more than virtually any country in the world, are British politicians getting into such a lather about Starbucks, amazon, google and all the rest? Three possible reasons: (i) they are simply ignorant (highly likely) or (ii) they are endeavouring to find a scapegoat for the country's ills or my favourite - both.