This isn't a mis-selling case because, as pointed out, the OP joined the pension scheme before he was given the impression that he could leave service, then marry, and still qualify for a spouse's pension; therefore this played no role in his becoming a member of the scheme.
What it did do was lead him to decide to defer his marriage. His claim is for negligent misrepresentation — "you carelessly gave me this impression; I accepted it in good faith and acted on the basis of it in a way that I wouldn't have, if you hadn't misled me; that action has cost me money."
If he were to fight this case, there'd be an evidential issue about proving that the misrepresentation caused the deferral. Was it a case that the church was booked, the reception was booked, the honeymoon was booked, and they were all deferred when the OP was told about his impending redundancy? Or that he and his intended to marry soon, but had no definite commitement, and "soon" just got extended a bit? The OP doesn't say, and I don't think we need to know; it's just that this would be a big issue if the matter went legal.
The other big issue is the fact that the decision the OP made had, in itself, nothing to do with pensions. It's one thing to hold a pension company responsible for a misrepresention that lead you to buy their product, or to exercise your options under the product in a particular way; it's quite another to hold it responsible for life decisions like marrying — decisions that they have no way of knowing that you are even contemplating, at the time when they are (carelessly) explaining the scheme rules to you.
A third issue is one that has already been pointed to — you have to look at the totality of the information that was given to the OP. The extract the OP has given above certainly looks like a very clear misrepresentation. Anybody reading that would certainly think that a clear distiction was made between (a) leaving service and (b) retirement, a later event, and that if you married between leaving service and retiring you would get a spouse's pension. But it's possible that other material given to the OP — like the scheme booklet — explained the situation more clearly and more correctly.