Tax laws are what's written in acts and statutes, but also how the courts interpret those laws.
When a company plans their tax they can either use the laws as they're intended (tax planning), they can deceive the tax authorities in order to illegally reduce their tax burden (tax evasion) or they can use the laws in ways which aren't necessarily intended, but which aren't yet outright illegal because we haven't had a court ruling yet (tax avoidance).
It's really easy to say "just change the law", but there are problems with that.
Prosecuting complex tax cases is fantastically expensive, and so tax authorities tend to only go to court when i) the case is obvious and ii) everything else has failed. (This includes for the small guy too -- if I chose not to pay my tax there are escalating steps of intervention before I get prosecuted). This means we simply don't know if a particular tax avoidance scheme is illegal or not, and we're unlikely to find out unless the tax authorities move to a more punitive regime.
And we don't necessarily want to give more power to the state. We want to limit the power of the state to some extent, which is why a lot of pretty unpleasant behaviours are perfectly legal, and it's why we impose that "beyond all reasonable doubt" requirement on criminal conviction.
Corporate values aren't (at least they shouldn't be) nonsense. A company could say "we do this because it's legally mandated and we don't have a choice", but that's the worst reason for doing something. Companies that make use of education, transport infrastructure, policing, healthcare, (all the things that are paid for by taxation) should pay their fair amount of tax, rather than using the law in weird unexpected ways to reduce their tax burden by an unreasonable amount.
There is a cultural thing going on here. No-one in the UK is saying that companies should go out of their way to pay more tax than they need to. And reducing their tax burden a bit is probably seen as okay. But we do have the concept of "taking the piss", and a lot of these companies are taking the piss. They need to realise that public tolerance for this is low and reducing, and that if they don't change their behaviours they're going to see increased tax regulation.