Mine is a BX05 batch of cars, like yours registered in March 2005, just a month before it all ended.
The service book has a stamp for MG Rover fleet.
I think they were in such dire straights, they were pre-registering like mad to get the numbers up.
Either that or pre-registered cars to get the numbers up for the plate change. Looks better for the numbers sold.
Think that's all there is to it.