obviously no longer enforced as when I google a location & click on Streetview many houses seem unnumbered following renovation etc
I suspect that it's no longer enforced because there is nobody left in Local Government to enforce it!
The worst case I ever found was along a lane linking two earlier Urban Districts. The boundary between them ran, for a distance, along the middle of the road and the lane was numbered independently from both ends - i.e. X Lane East and X Lane West.
Following the convention of odd numbers on the right hand side of the road (or is it the left - I'm not sure but it doesn't matter!), I found that there were even numbers on both sides of the road which bore no relationship to one another
Assuming "X" is "Ack" I walked along that road the other day and noticed the same phenomenon.
Various generalisations above. The idea of "clockwise" numbering, or number 1 always being on the left as you face up the road, is not universal. Another tale I have seen mentioned is that number 1 is at the end of the road nearest the sorting office/post office. Never seen proof of that one.
The quoted power of local authorities to require numbers to be displayed, is routinely ignored by shops especially nationwide chains. I was told once that only certain authorities ever took up that power, and if the retail giants have any interest in displaying their number, it is only in those areas (I expect Westminster is a key one) that enforce it. And as already said, the councils no longer have the resources to enforce stuff like that.