Classic! Dell (allegedly) selling hardware that can't run the operating system that's running on it... happened with a machine I bought a while back for Vista. Luckily I could get some extra RAM installed in it.
I think you've got a case here... if the spec. that you've got mentioned bitlocker, you've really got a case. Have you any email history with the sales person? What about the spec. that appears on their website for your machine? Does it mention bitlocker? If so - IMMEDIATELY take a copy of this page so that you can hold on to it... and then let them know you've proof that they are offering this as part of the product.
They might classify bitlocker as being an optional add-on... but since it ships with the operating system, I really think that the hardware needed for the component should be included, if they are going to include this in your operating system.
Is this not "trade-descriptions" where you say that a particular laptop can run an operating system... when it turns out that it can't run all the components of that operating-system?
I think if you make enough noise with them, they will hopefully give in.
If not, I'd recommend using a third-party drive encryption system... there might even be some open-source or free ones out there...