Incorrect. There was no act of Parliament that said we had to sit down. So one isn't required to allow standing to happen.
It was a Government regulation not legislation that was introduced.
At the time of the Taylor Report it was originally stated all 4 top divisions would be all seated, but after a review (after criticism of the regulation), the then sports minister David Mellor stood up in parliament and simply stated it was be changed to the top two divisions only (and phased in when clubs got promoted etc) - but that is all it would take, a Government review and an announcement in parliament repelling the legislation.
That is why it is not against the law to stand up because there is no law. It is a stadium regulation to remain in your seat, but it is not the law of the land and that is why the rule is unenforceable through the courts (yes of course clubs can ban you) but you can't be taken to court for standing because you haven't broken a law.
A steward can throw you out because they are there to enforce stadium regulations, but a policeman can't throw you out just for standing, because you haven't broken any law.