Impossible.
Around 800,000 L tests are conducted each year and the average waiting time is around six months. The UK has 42 million full licence holders, a retest every five years would mean another 8.2 million tests being conducted, ten times the number now. There are nowhere near the number of examiners, test centres and booking staff to cope with that and there probably never could be.
On top of that, as an ex ADI I don't think retesting would have any effect on driving standards at all. How people behave on test conditions and then in the real world are completely different.