The visa is not valid until the date shown on the visa. He can enter the UK on or after the "valid from" date, but not before.

 

I think you are referring to the tourist visa, which you can get on the spot. No, that would not be a good idea, because the tourist visa cannot be converted into any other visa.

 

I thought it would be entering the UK under the Visa Waiver Program (i.e. visa-free tourism).

But the conclusion is the same: if you enter as a tourist, you're a tourist. m8nda's fiancé needs to enter as a fiancé in order to be allowed to a) stay beyond 90 days and b) marry. It would cause some complications at UK immigration to justify entering as a tourist with an inactive fiancé visa glued into the passport.

I'm not sure whether it would be acceptable to enter as a tourist, leave for the Continent for a week (again visa-free as a tourist) then re-enter the UK as a fiancé from Europe rather than direct from the States. For me it would be an expensive gamble to risk being turned away at UK immigration for the sake of a technicality and a couple of weeks...