The company have to honour agreements given to the tax office by way of deed up to the time that they can renegotiate a new agreement. They can dispose of the current liability by going bust and letting the PPF pick up the pieces or by buying everyone a deferred annuity to cover current liabilities. A mere £4Billion.
To ammend the deed they need to now negotiate with the trustees and provide an extensive information program to scheme members.
So it is perfectly legal to garuntee present benefits and ammend future benefits. They cannot make changes to already earned benefits, for that you could sue.