In a court of law if pricing is wrong it is usually down to whether it was "obviously" a mistake.
I'd love to see that law.
In a court it's what the contract says.
A mistake being obvious or not has little to do with it unless such a clause is in the contract.
With a physical shop they do not have to honour a misprice on the shelf, ever.
Once it's purchased though it's yours, contract complete.
With an online retailer it depends what their t&C's say.
With Amazon the contract is complete upon dispatch. They don't recall deliveries if they notice a mistake.
With John Lewis contract is complete when you receive the goods.
They do recall items out for delivery if they notice pricing mistakes.
If BT have in their terms and conditions that they have the right to cancel the order before it goes live (they do have this) it matters not how obvious an error it is.
BT have zero obligation to honour this deal.
Credit to BT for honouring this deal.