A CAA prosecution is highly unlikely, and if it was attempted it would likely fail, for the reasons given above (the CAA rep nodding through the presentation, etc). The CAA wouldn't even try it in the first place, IMHO.
The CAA read these forums daily and no doubt find all this highly amusing. They might just possibly be wondering how the hell did they create this mess.
A prosecution is also unlikely because the stuff is required only when flying too far from land or, in the case of oxygen, above 10k feet. The evidence for the former would probably have to be a radar track and they aren't kept for ever. The evidence for the latter likewise if VFR and with a Mode C transponder but again radar tapes are not kept for ever. On an IFR flight plan filed for FL150 it might be a problem...
IMHO the real issue here is airworthiness of the aircraft. If you have mandatory but non-approved kit then the CofA is invalid. Again, a CAA action on this is unlikely (reasons as above) but the insurance would quite likely be void. Insurance does cover pilot negligence (contrary to what most people on pilot forums say) but it doesn't cover flying with a void CofA. What I don't know is how this is affected by the fact that the violation occurs only if over water or above 10k ft. In effect, you have a valid CofA but it ceases to be valid over water or above 10k ft.
In the FAA case (which the ANO doesn't apply to) I am sure that with say no oxygen on board the CofA remains valid but not using o2 as prescribed is a violation on the pilot. Not having an ELT is a CofA violation however, because on an N-reg it has to be carried continuously.