Try emailing Ford U.K to start they may have some records of when it was first delivered to a Dealer

There are a few possibilities.
It may have been built and not left the factory immediately.
It may have been put into storage by the factory locally.
It may have been shipped by the factory and got lost in a compound along the way maybe at the docks or nearby, they deal with 10's of thousands of shipped cars so loosing track is not that difficult.
It may have been delivered to a Dealer and stored there on behalf of Ford, maybe they also lost track of it.
And as above maybe they weren't selling well at that time, but Ford kept producing.

It also may have been bought by a customer who for some reason chose not to register it immediately.
It may also have been first registered abroad or Jersey / Guernsey / I.O.M