Well yes the prices will go down after official release of the 2018 CT. But until that time, the prices should stay the same and therefore are irrelevant unless you're going to buy later on.
Should be between £7.5k to £11k dependent on spec, mileage and whether it's a private/dealer sale. I certainly wouldn't pay more than £11k anyway as there are some 2012 ones with 30k for that much