You can sell up and settle at any time. The thing is that you have to pay off whatever settlement figure they give you. So if the car is worth less than the settlement figure then you'll need to find the difference from somewhere else, eating into your £2-3k BMW no doubt. If the car does have enough equity on it to pay off the finance then you're laughing, but something tells me this won't be the case. In the first 6 months of the deal you'll be mostly be paying off interset on the loan so the sellement figure won't have dropped as quickly as the value of your Dooper.
A clause you may wish to look into is the handing of the car back with nothing to pay once you've paid 50% of the finance. This will be somewhere between 12 & 18 months into your 2 yr deal. If this is of interest it may be a better option than buying a car for 2-3k now that has no warranty and an increased likelyhood of maintenance outlay, MOT failure etc. Depends why you want to swap your Dooper really.