I think not, DD. As with many such issues, you have to read all of the documentation instead of picking out the bits that suit the argument you are trying to win.
Firstly, the guidance you quote from the H of C library only applies to a very few areas in the country which are not part of a road safety partnership scheme. (However similar guidance has been issued to them and later consolidated into a DfT circular). Secondly, it is said that "The guidance represented good practice which Partnerships were expected to consider, but was not mandatory." That is, it did not (and does not) form part of the law covering speeding offences and their detection by automatic and manually operated devices. In fact, the document itself (and, I believe the subsequent guidance issued to safety partnerships) says this:
"To be clear, in terms of enforcement, the guidance has no bearing on the enforcement of offences. Non-compliance with this guidance does not provide any mitigation of, or defence for, an alleged offence committed under current UK law”.
To mount a defence against a speeding offence on the grounds that the detecting police officer was not visible enough (according to the guidelines) would be bound to fail. The guidelines themselves make it quite clear that they are guidance towards "best practice" and cannot be used as a defence. The defendant would have to show that non-compliance with the guidelines either encouraged him to commit the offence or that the process of gathering evidence against him was seriously flawed or impeded by non-compliance with the guidance. I think he might struggle on either count.