In a recent thread on Amazon, I had a look at what Amazon were paying, and what they were avoiding, and it appeared that Amazon pay about two thirds of the notionally "full" tax bill. In aggregate national terms, one third of tax liability is business rates, one third is corporation tax (the bit you can try and avoid), and one third is employer's NI. So if you can avoid most of your corporation tax, then you'd avoid roughly one third of the taxes you might otherwise incur.
Obviously the detail is a lot more complicated, but that's how it looks to pan out for Amazon with several thousand UK employees and UK distribution centres. Google and eBay I'd expect to be different because there's no physical delivery, and arguably they'd be proportionately worse in tax evasion (if following the transfer pricing tax avoidance model) than Starbucks or Amazon because these two can't avoid the payroll taxes or rates on its stores, whereas Google and eBay probably don't have much in the way of UK staff or premises.
Taking Google, they are reported as paying £6m of UK tax in 2011. According to their investor presentations they make 10% of global revenues in the UK, making total UK revenues around $3.7bn. Group pre tax margin is around 33%, so that implies UK taxable profit ought to have been around $1bn (after losing say $0.2 or 0.3bn for tax deductibles), and at prevailing tax rates that would have further implied a UK tax liability of $260m, say £170m. So by Google's standards, Starbuck and Amazon are tax saints (although only because they can't avoid the rates and payroll taxes).
IBM and HP do employ more people in the UK and will pay more NI, and more business rates, but they've been critcised for avoiding US taxes, so I'm sure they won't be paying any UK taxes they can avoid. Moreover they encourage their customers to offshore work, so avoiding all UK taxes, and removing jobs from the economy.
So in summary: It's a mess, it loses billions in tax revenues, and motivates all the wrong behaviours. Luckily David Cameron has a finger on the nation's pulse, and is turning his incisive, Sauron-esque attention to the matter of gay marriage.