You make some good points SineadHanks, and this might have been a good idea on paper but I am afraid that in the crazy world of the public sector it was doomed to failure. Specifically (your points in bold):
What if you are looking for 'nearest' which is close to two seperate local authorities?
Yes that is a bit of a pain, I live within 1/2 a mile of three LAs so I had to contact each one separately to get a list when I was looking for a CM. That was not really the end of the world though.
It's an old fashioned myth that 'entering postocdes' necessarily costs money for LAs...UpMyStreet pioneered this quite some time ago.
Yes, in fact any half-decent web programmer can set something up in an afternoon that will do this. That is not the hard part though...
These days it's easy to set up a system which automatically updates your own and an external website.
Ah yes, given a blank sheet of paper and a responsive IT facility it is. Unfortunately many LAs are tied in to systems they have bought/leased/subcontracted to provide their CM information and found it impossible to effectively link that in to the central system. Some have set up parallel systems, some have gone out to tender for replacement systems complete with updates to the centre, some have asked their existing suppliers to amend their systems with varying degrees of success and cost...
I believe there are about 350 local authorities in England and Wales. If each one of those has spent £15,000 on this project that would be over £5m. How much was spent centrally I wonder? And all for nothing :(