I'm a bit confused - are you saying that if you go for this new deal, you would have £450 leftover once you've paid what you have to on both mortgages and the car loan also?
So the £450 is available to over-pay.
That could cushion an enormous rate-rise.
What happens with your overpayments too? Some banks will let you use these to allow payment-holidays later on.
As the interest rate is so low though, I'd recommend NOT overpaying on the mortgage. DEFINITLY overpay on the car loan and get rid of it asap.
Once you have done that, if interest rates are the same as they are now, put the money you have for overpayments in an ISA - it will earn more interest than the mortgage is charging you, and will be available to do whatever you want with at a later date (eg. if you neeed a new car you could avoid a car-loan).
If rates increase sharply then you can overpay a chunk to minimise the interest you are paying then, but in the mean time will be earning more money in the savings account.
Interest rates are currently forecast to stay low for another year or so. In my opinion you would be daft to fix now and waste money - that rate is a gift!!
I was in a similar position 2-3 years ago (just before rates plumetted) and went for a base-rate + 0.89% for life instead of a fixed-rate that would be have cost is the same at the time.
You can think of any gradual rate rises as just starting to balance out the difference between this bargain and what a fixed-rate would have been costing you over the cheaper period.
HTH