I am not a lawyer - caveat -
Yep, I think the general idea is "invitation to treat", ie they can put a price on something, you decide to buy, and then the contract is only then fulfilled. So if a mistaken price is put, ie a car for £10, and you then decide to buy, they can still refuse to sell to you, as it is only an invitation, which they may withdraw from you, when it becomes apparent the car is not £10.
Once you have made the bargain with the retailer, ie they have invited you, you have agreed and a contract is created, at which point it is then up to them to perform to their contract with you.
If they fail to perform the contract, you may sue them for a specific breech of contract, to provide goods to you, for the sum agreed, for which payment has been taken.
The appropriate measure of damages you are entitled to for their failure is likely to be limited to "put you in the position you were in, prior to the breech of contract". Ie, they refund your money in full, and you are as it were, back in funds, and without a shed. Unless you had a speific need for the shed, delivered on that day, and its non arrival has caused you further quantifiable loss, then thats that.
So, I'd say - "bring the shed, its the one I wanted, but I want free delivery to make up for my inconvenience", which is a valid head of claim in law. Mattocks Vs Mann recognised the time taken in phone calls or letters to sort out a litigious matter.
Of course, there might be other consumer legislation I am not aware of to ammend this view.