I think it's an excellent perk; well done!
The car allowance will be paid along with the monthly salary; it will be included as income and will be subject to 40% tax (as you said your husband is a higher rate payer). So you will probably end up with around £400 net to use towards a car and related expenses, which is still a pretty good perk.
Getting a general car allowance usually means you have to pay your own car costs; tax, insurance, servicing etc and you also often receive a lower mileage rate than non-car allowance staff (as your company are already contributing towards 'wear and tear' on your car).
I've had company cars and also separately car allowances, and my preference has always been the car allowance.. more versatility, you can spend it on what you like (within reason) and you get to actually own the car.
One of the expectations from my firms mean that getting a car allowance also meant for that I would be expected to drive to various locations (within a limited distance) rather than get a train, cab or hire car - so if it's the only car for your family, you'd have to expect the working partner to use it when required.
I have known colleagues NOT to buy a car even if they receive car allowance, which is fine for example if you lived in central london and didn't need one.. they only thing those colleagues had to do was hire their own car or pay for their own train travel and and when was needed.
Hope your relocation negotations go well.