I don't have the current regulations, only BS7671:2008 chapter 42 does cover thermal effects, however I can see no reference to wood, and plastic would be effected in same way as wood, I know dad's old house did have MK wooden back boxes which were renewed but mainly to get fixing screws to fix switches.
The point is however if old enough to be wood, then likely pre 1966, which was date when there was a major change in the regulations, from that date lights required earths.
I hear many times the view there is no need to upgrade just because new regulations now no longer permit what was installed, however since the issue of "Rules and Regulations for the Prevention of Fire Risks Arising from Electric Lighting." Issued in 1882 there have been many changes, back then knife switches were allowed with exposed live parts, so there has to be some where a cut off point.
In 1992 it became BS7671, so one could say that was the first edition? Also it did say on the old PIR before renamed EICR as one of the codes, complies with previous edition or some thing similar, and the previous edition is now BS7671:2008.
Law wise not really sure, the electricity at work act likely includes private houses as at some point people who are at work, need to access the house, be it a carpet fitter, social worker, police, fire, customs, or local priest. As to children I would think the law also covers them, you are today allowed suicide, but manslaughter is still against the law, so leaving the home in a dangerous condition is not permitted.
As to regs not being retrospective, well they are not law anyway, so there is nothing to say anyone other than a scheme member must follow them, scheme members sign a contract with scheme provider so not to follow BS7671 would be a breach of contract, but there is nothing to say when I work for local railway that I must follow BS7671, there are special railways rules, but as long as I don't do something which causes a danger, there is not really a problem, however easiest way to ensure I don't do some thing to cause a danger is to follow BS7671.
So the home owner wants to be safe, forget laws and regulations, he simply wants to be safe, so there are a number of points.
1) No rubber insulated cable which is degrading.
2) Earth to lights, or failing that, all class II switches and light fittings.
3) All earths actually sound, there are many premises without an effective earth due in the main to plastic water pipes.
4) Over current devices are matched to cable, i.e. ring final is actually a ring, and a MCB has not been changed to allow a larger shower without also upgrading cables.
5) Any fault will where possible result in an automatic disconnection of the supply, so the earth loop impedance is low enough so either a fuse, MCB, RCBO or RCD will automatic disconnect the supply if there is a fault.
6) No exposed live parts.
7) Cables are routed in safe zones so unlikely to be damaged with other work on the home.
In the main most homes will have some place where some thing is not as we would want, some items are easy to test, like the ring final actually being a ring, others like safe zones, we really have no way to know if cables actually run in safe zones or not. As yet not seen anyone doing a EICR actually tracing the cable routes.
So silver coloured cores either tinned copper or aluminium, the use of the latter was very short lived, Ian Smith Prime Minister of Rhodesia from 1964 to 1979 resulted in a shortage of copper, not sure of dates when aluminium used, do remember starter cables on wagons exploding due to being made of aluminium, would not think any now left in service. Nothing wrong with tinned copper, but likely tinned copper is in rubber insulation and that is a problem, early plastic also had a problem with the plasticizer leaching out, and they were OK if not moved, but working on the cables they could crack, Australia had a problem with some cable sold by Woolworths and it had to be replaced, I would assume mainly due to heat in Australia here we some times see a green slime coming out of the cables, but not heard of it being changed just because of that unless also damaged.
To my mind the EICR has a problem in that it lists items which don't really need addressing, fuse box made of wood and bakelite are still serviceable, but you can't in many fit over a 32A fuse/MCB some did have one slot, but unless the special 40A MCB could cause the contact to overheat and burn, also can't fit a RCD inside the box, so I suppose there are three things we look for.
1) Some thing has degraded over time.
2) Some thing has been modified so does not comply with original regulations when installed, or was never installed correctly in the first place.
3) Modern equipment could be installed to increase the safety. Be this be fire resistance or an earth leakage trip.
The main change over the years has been the distribution unit, it started as a fuse box, then it was realised the wrong size fuse cable could be used, so went to cartridge fuses where in the control of an ordinary person, it then allowed use of MCB which is like a fuse but does not need replacing, you can reset it, then we included a earth trip, first outside the distribution unit, then integral with distribution unit, then we went to type tested distribution units called consumer units (CU) when in the control of an ordinary person, then we went, well still going to RCBO which combines the overload and earth leakage into one device, and finally went to metal consumer units.
In the main any home wired before 2018 will not comply with latest edition of BS7671 because of the fire requirements, and swapping the CU is the single thing that will increase home safety in most cases, however it also causes the most problems, cables which would work OK without earth leakage will often trip the new trips, specially when many circuits are put on one earth leakage trip (RCD) so before changing the installation needs testing.
The testing is a problem, as we as electricians are not allowed to switch off power without the owners permission, but also can't turn it on again if the circuit is dangerous, and to test circuits must be switched off. When I came to buy this house, before buying, only a visual inspection was permitted, and the guy made some mistakes, wrote on report new CU fitted, old one still in place in the ceiling, in fact old one was still in use, new one was an addition.
So you want some one who uses some common sense, and will give you some advice, without sucking air through his teeth, often people want to know options, including a plan so for example, fit new CU this year, and rewire in a couple of years time, just before you re-decorate.
