"Do you really think that Vodafone plc completely avoids paying all tax in the UK?"
Yes, because they happily admitted to doing it for years - putting aside the tax they legally owed in this country in offshore accounts so that they could "work" it until it was asked for. Off the top of my head, they did this for something like six years before HMRC got up the courage to approach them about it.
Where the tax was "earned" is irrelevant (most of it was on-shore anyway); the laws on this country required them to pay that tax.
"Do you believe that one man's decision to "let them off" would be vindicated by the Chancellor, the Treasury, and the Supreme Court, if they actually legally owed £4/5 billion?"
Again, yes, because no one looked in to the deal at the time. Dave Hartnett was forced to declare how cosy the deal had got /after it had all been signed off/ (it was a little more than a few lunches).
His job was to chase and close large tax avoidance schemes (to rhyme with scams) such as what Voda were doing. True, there's the possibility that the contract will be retrospectively annulled, but I won't hold my breath.
Voda ended up agreeing to pay less than a quarter of what they legally owed (and knew they did), but DH let them off the rest and considered it a "good deal" (his words).
They haven't covered it in a while, but the Private Eye did some very in-depth coverage in to what Vodafone owed, why, and how they got away with it.
Not that they are the exception, by any margin.