This is exactly the problem you are having though. The method hardly matters at all. What's relevant is merely the principle. It is the principle that has been commented on by several people, with regard to your apparent lack of understanding of it.
No, it doesn't. Or not unless you believe you are entitled to purchase[1] a television set for £0.01 when Amazon periodically screw that up (you in fact are not, for avoidance of doubt).
It's an invitation to treat. There is no obligation on either side to transact at that price. I do agree that a TOC would have a job pushing anything other than a return to Standard if the payment of the correct fare was to be declined, though.
[1] That has a slight complication in that if they accept the order and take the money, you have more rights *unless the amount seemed obviously silly*. This isn't the case with this Weekend First example, because it's simply an invitation to treat, no contract is formed until the transaction actually occurs.