Well if you have the honest held belief that they'll give you the roofbox as compensation...

Surely the fact that both times it's been exactly the same mileage suggests it isn't being based on quality of the oil (variable).

In all the cars I've had on variable servicing, not one of them has ever done exactly the same mileage between services unless there has been an extra oil change between and they have therefore given maximum miles possible between services.

I'd suggest if you don't get anywhere with the dealer you should inform the dealer you will look at the small claims court as I would think that to most normal people the "evidence" you have would suggest to me on the balance of probability that they put your car on fixed servicing.

Sounds to me like somebody in service put your car on fixed either by accident or in an attempt to make more money from servicing in these hard times. Normally I'd say it was just an accident or mistake, but the way you have been rubbished, well you can't rule things out totally.