That's not really accurate - they merely interpreted ambiguities in the tax law in the most favourable way for themselves (and paid all the tax that was due... In Luxembourg, another EU country). Google do this (in Ireland). Hell, The Guardian, the most left-wing paper in the UK, does all its financial affairs offshore (really: http://order-order.com/2009/02/02/guardians-tax-hypocrisy-is...). They pay 0.3% of their profits in tax. 0.3%.
A similar case is ongoing now (Gregg's the bakers) but bizarrely public opinion seems to have come down on the opposite side this time.
As far as I understand it, the British tax code has clauses to the effect of "business structures created for the sole purpose of evading tax don't count". In that regard, they didn't follow the rules.
A similar case is ongoing now (Gregg's the bakers) but bizarrely public opinion seems to have come down on the opposite side this time.