I think it has something to do with the fact that dealers in the UK place stock orders and you may be lucky to find one that has not been allocated.
Here in Spain, apart from the demo cars, they tend only to order from the factory to the customers exact requirements. As a result the delivery on the 1.4 is over 5 months and in some cases longer.
Let's hope the dealer is right, but you won't know until you get a temporary date from SEAT.