(Answering my own post ... )
I still haven't found any reference material about UK Codes of Practice, or links to anything that says what reasonable assurances of quality we should expect for work on external domestic (signal) cable installations ... but I have found these two accrediting / licensing organisations -
Confederation of Aerial Industries Ltd. (CAI)
Registered Digital Installer - Licensing Body (RDI-LB, or as they invite us to use - "rdi-lb")
These rather good organisations should help me find a capable and high quality local company to "make good" this tatty wiring job mentioned above.
I note all the promises and plaudits that the CAI and rdi-lb give about their respective member companies and how we can expect delivery of "quality reception" and "good signal strength", but I'm disappointed that I can't yet
see any mention on their websites of the "differently-important", less-technical end-products of such installations, namely secure, neat and as discrete-as-possible cable runs, removal and disposal of any replaced components, and the avoidance where possible of damage to stone-work, or to rendering or to woodwork.
I'm sure there must be something
in their accreditation and assessment criteria about these aspects of their member companies' work! It's just a pity it seems so hard to find ...