Presumably, big corporations are not only avoiding tax via legal means, but they are creating these legal means to avoid tax. Why do it illegally, when you can bribe/buy politicians, who then pass laws and allow you to avoid all the tax you want.
It could be 100% legal (not bribery specifically, but campaign financing for example), but i don't think most people are really okay with it.
I'm perfectly okay with someone finding a legal loophole, i am not okay with them blocking said loophole, or introducing new ones.
HMRC has been notoriously lax in enforcing and collecting tax nominally owed by the largest corporations - most recently Google, which "negotiated" a payment of £130m in back taxes due on £4.6bn in sales.
The details of the "negotiation" have been kept confidential, so we have no way of knowing how they were calculated.