mylilangel:
This is a bit difficult to answer as it depends on alot of details.
You should phone the Halifax to get advice and a quote to see roughly what the cost would be. They can probably give you a quote if you know which year/model you are buying, without the reg no.
I don't know the way in which Halifax operate. Can they add the new car to your current policy and charge a pro-rata additional premium? Or do you have to have a new annual policy for the new car? If so what 2nd car/policy discount do the Halifax offer. You obviously want to maintain the current policy until January to get the 4 yrs NCD. The Halifax can transfer the NCD to the Honda if this makes sense. The Honda is not necessarily more expensive to insure that the vectra, just because it is newer. Halifax can give you quotes to see which car you would be better having the NCD against to obtain the cheapest premium. If Halifax are not competitive for the car left without NCD for a new annual policy, get other quotes.
If you end up with separate policies, with different renewal dates, you would have to check whether it is cheaper to cover with the same Insurer or not. If you have 2 policies with an Insurer, they will often allow a 2nd car discount, but whether this is cheaper than having 2 policies with different insurers you will have to find out. It depends on what their base premiums are and what 2nd car discount amount is.
Alot of cars are jointly owned between husband and wife. The main driver should be the policyholder and the V5 registration should ideally match, showing the same person as being the registered keeper.
Hope this helps.