You're right, Drummond.
They're phasing out an old technology and migrating people over to the new network. Every cell company tries to move their customers over before they shut down the obsolete network but there is always customers that are happy with the old one and have to be forced to change.
The communications companies can't be expected to maintain separate networks because it's cost prohibitive and slower besides.
That ASDA network sounds like the British version of the CDMA technology that's being phased out here in the United States... just a guess on my part, I'm on my phone and can't look it up.
Tricky Dicky, the new technology is 4G and it's worlds faster for handling data. If you don't use data on your phone, you probably won't see much difference but it sounds to me like they have been trying to get you to upgrade for a while now and acted properly.
It's not 'bad engineering', it's new technology evolving.
Interdum feror cupidine partium magnarum Europae vincendarum