Sale of Goods Act is in your favour and if you want a refund you can get one. I had this problem when I bought my 172 and despite the dealer claiming they could not offere a refund they changed there mind when I threatened legal action based on the SoG Act. As it happens I kept the car after the dealer offered to cover all costs to make my 172 UK spec.
They have a duty to inform you if a car is imported, they cannot say afterwards that there policy is not to tell you one way or another.
You have the HPI which is inaccurate, that is enough to prove the car does not match the description on which it was sold.
Sale of Goods Act
Under the Sale of Goods Act 1979 traders must sell goods that are as described and of satisfactory quality.
If consumers discover that products do not meet these requirements they can reject them and ask for their money back providing they do so quickly. Alternatively, they can request a repair or replacement or claim compensation.
http://www.br0wn.co.uk/lawofshopping/
What to do if the Shopkeeper Refuses a refund on faulty goods
Some retailers actually don't know the law, but much more likely they're just hoping you don't.
First, ask to see the manager or supervisor and tell them you are claiming your rights under the sale of goods act. 75% will give in at this point.
If you still don't get joy, phone, email or write in to the manager or the head office (if its a chain store). Tell them your complaint, and that you want a refund in line with your rights under the Sale of Goods Act 1979 (and subsequent amendments). Also say that if they don't give a refund, you will pursue that matter in the small claims court. This should wrap up another 20% - you're left with the rare few who will resist this pressure.
Going to small claims isn't actually that hard, if you need to go this far, your first stop is the Citizens Advice Bureau.
Sometimes, the shopkeeper may say its not his problem and you should contact the manufacturer. This is wrong. You have a contract of sale with the shopkeeper. He has his own contract with his supplier, its up to him to claim for faults under his contract.