Swap over any current Insurance to the new car from 1/3/12. Get some quotes from companies to see who comes out the most competitive for covering the cars.
If you will not be driving the old car after 1/3/12, I thought that you could declare it as SORN. The governments website confirms this is the case. It is only if you have a vehicle with a current tax disc that can be on the road, that you need to have Insurance in place.
Your best bet is to speak to a decent local Insurance broker and to explain the position, The should be able to guide you, as to what the best option is.