It's illegal to sell a vehicle which is unroadworthy (unless it's made absolutely clear that the sale is 'for spares or repair'). However, a current MoT certificate is not proof of 'roadworthiness' and the absence of one does not mean that the vehicle is necessarily unroadworthy.
Vehicles which are owned by a registered motor trader do not require a road tax licence, and hence don't require an MoT certificate. Similarly, any vehicle which is is the subject of SORN doesn't require a road tax licence (and thus doesn't need an MoT certificate).
Since you didn't know the history of the vehicle (i.e. whether it had been subject to SORN, or held by a motor trader), your assumption that it would have passed an MoT test in May 2008 is completely illogical. Unless the dealer implied that the car had a current MoT certificate, he's done nothing wrong.
One of the most basic questions when buying a secondhand car is always "How long has it got left on the MoT?". If you failed to ask that question (and the dealer didn't imply that the vehicle was sold with a current MoT certificate) you've only got yourself to blame. The legal maxim of caveat emptor ("let the buyer beware") applies.
Chris