Where do I start:
If you keep using your car to drive for walks, you will need petrol, where you will come into contact with people needlessly, and you will touch the equipment, increasing the risk of spreading.
If you use your car, you may break down, or have an accident, neither of which you can predict. Emergency services may be needed, putting unnecessary strain on the NHS. You may need assistance from people towing your car, looking at the engine, etc etc. All unnecessary contact and strain.
Depending on how far you have driven, you may go into a rural area which has more limited resources than where you live. Not as many hospitals or doctors. If something happens on your walk, such as a fall or a funny turn, and you need medial attention, you will be putting pressure on already very limited resources.
On your way back, you may decide to pop to the shop - which of course is allowed. However, you may decide to go to a different one, in a different area. If you are an unknown carrier, you have spread the virus into a new area. And what for? For your nice little walk.
Need i go on? I live in a beauty spot, I'm seeing all of the above on a regular basis. Just don't do it! Why can't everyone just do as they're bloody well told for once! There's always someone 'above the rules'.