//I think the idea of stopping people travelling any distance in cars is to prevent them taking the virus from one area to another.//
And it's also to lessen the possibility of having a Road Traffic Accident where the emergency services and medical facilities will have to treat you rather than virus victims.
There are many aspects of this lockdown that have not been properly thought through and seem to have been written on the bag of a fag packet over a pint in the pub (before they were closed down). They are not going to be able to keep people at home in this way for any length of time. Come May or soon after people will have been locked down for six weeks or more, the weather will be considerably warmer, the days longer, the infection rate will still be on the up (because unless they choose to die of starvation, most people simply cannot help but mix with others) and many people will be asking themselves what the point of it all is.
Then there is the point made by DeskDiary. The economy by that time will be in absolute ruins, many small businesses will have gone to the wall never to reopen. The current strategy means that, one way or another, this will not end well unless the virus simply "dies out" - and there's not much to suggest that is likely.