You may live in a safe area but you must at some time drive through unsafe areas. Often you won;t know it until someone pinches your hub caps.
Getting hijacked is an extremely remote possiblity. It's not in the UK crime culture (ok it happens, but only rarely) and also, if you're wearing your seat belt, they couldn't pull you from the car. The biggest risk is someone reaching in and grabbing your handbag or any parcels. This used to happen a lot in Slough about 5 years ago, where the illegal immigrants imported the practice and the local yobs copied them.
It is Not a good idea to decide to lock when stopped and release when moving. I can guarantee that you'll forget to do this sometime.
You should be aware that at traffic lights bad people will open a door and get into the car, or reach in and nick your handbag etc., in the space of a couple of breaths. It's amzingingly fast and you wouldn't react in time to stop them.
Also, it may considered to be driving without Due Care if Mr Plodd spotted you doing it at the traffic lights. (and playing with the central locking button!).
If you "lock" the doors, but not "deadlock" them, every time that you use the car then you'll be safe enough.
A door which is "locked" but not "deadlocked" cannot be opened from outside but can be opened from inside.
See your car manual for more information on this.
Normally "locking" means pressing the key fob button (or internal central locking button) just once, but "deadlocking" involves two pressings on the key fob or internal central locking button.
That way you and your passengers will still be able to open the doors from the inside if they have to escape in an emergency. Also, if you're incapacitated or unconcious then the fire brigade will just cut open your door to get you out anyway.
This time of year your biggest danger will be from drunks and selfish brain-dead, inconsiderate driving . I've seen more on Boxing Day this year than in the last six months combined.
Be safe and have a Happy New Year.