There is what you should do, and what is reasonable to do, not quite the same, the regulations
BS 7671:2008 411.3.1 said:
A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a
lampholder having no exposed-conductive-parts and suspended from such a point.
for low voltage installations (230 volt) but extra low voltage (12 volt) do not need the "circuit protective conductor" better known as the earth.
The rules changed in 1966, and GU10 lamps are not suspended like the normal pendent, so an earth wire is required to be run to them, even when there is nothing to connect it to. I have seen videos of inspectors do an electrical installation condition report (EICR) removing lamps to check, personally I think that is going OTT, and a little common sense should be used, it does seem a little pointless rewiring with an earth when not used, however I am sure that is what was also felt in 1966 when the rules changed.
The dichroic reflector used GZ10 lamps, and the GU10 had a bevel on the socket to stop GZ10 lamps from being used, there was also the L2 dimple which was designed so only energy saving lamps can be used, however that idea seems to have been abandoned.
The G5.3 however had nothing to stop wrong lamps being used, hence the warning. The rules stated
422.3.1 Except for equipment for which an appropriate product standard specifies requirements, a luminaire shall
be kept at an adequate distance from Combustible materials. Unless otherwise recommended by the manufacturer, a
small spotlight or projector shall be installed at the following minimum distance from combustible materials:
(i) Rating up to 100 W 0.5 m
(ii) Over 100 and up to 300 W 0.8 m
(iii) Over 300 and up to 500 W 1.0 m
NOTE: A luminaire with a lamp that could eject flammable materials in case of failure should be constructed with a safety
protective shield for the lamp in accordance with the manufacturer's instructions.
A little consideration and one can see near impossible for lamps to be fitted in the ceiling as very few beams are 0.5 meters thick so lamp would be too close to floor boards of room above, however there were some 75 watt MR16 lamps, but in the main limited to 50 watt, again some common sense must be used. The hoods used are in the main to stop fire reaching upper floors too quickly, not to stop heat from bulb. The plaster board stops fire spreading, cutting holes in it clearly means fire can get through the holes. Having a metal and glass lamp likely will stop fire nearly as good as the plaster board, although with plastic LED lamps maybe we should reconsider?
The G5.3 MR16 lamp was originally powered by a toroidal transformer, an expensive heavy lump which did not regulate the output, the output voltage was a ratio of input voltage, and the quartz lamp needs the quartz to be that hot that the tungsten will not stick to it, but not too hot so it burns out, so the toroidal transformer was replaced with the electronic transformer which was a device that contained a transformer but also changed the frequency so it could be smaller, typical in kHz range, and it regulated the output, as long as output was within set limits, often marked 40 - 100 VA (VA is nearly the same as watts) and so a lamp/s under 40 watt would fail to work. But the quartz lamps lasted longer due to better regulated supply. Some latter power supplies could run from zero, so no all switch mode power supplies need a load over a set limit.
The use of electronic transformers however did mean the cable from the transformer to lamp needed to be limited in length, often 0.5 meter was stated, and with a khz supply fed into a diode it could very easy become a radio transmitter, so many LED bulbs were marked 50 Hz, often they will work with kHz and DC, but without opening a lamp and seeing what is inside, this is simply guess work. Many GU10 lamps will only work with 50 Hz as they use a capacitor as a volt dropper so frequency matters, but not seen this with G5.3, however some G5.3 are designed for DC only and have a huge voltage range typically 10 - 30 volt, mainly for caravans and boats, but one can't make general statements, as there are exceptions.
The old quartz should not as already said be dimmed, however it was common to find power supplies designed to be dimmed, not a clue what the thinking was there? Some LED can be dimmed, others can't, but when dimmed the colour does not change, so not the same ambiance, so with LED the "Smart" bulb is way to go so not only dimmed but also colour change, I have them in my bedroom but rarely bother changing output or colour, bit of a waste really.
Converting G5.3 to GU10 means instead of using a rubber sucker to turn the bulb to remove it, you need to release a spring clip and drop the bulb and connector then remove the connector so a lot of messing around, I do use GU10 pods so I can direct light to where required, i.e. read a book in bed, but not really into making a room look like a planetarium, I am sure arranged in the great bear would be a talking point, but really more interested in lighting the room.
As to MR16 there are few LED lamps which have a multifaceted reflector or though still 16/8th inch across, so without the reflector not really MR16 bulbs, just MR16 compatible.
However the main problem seems to be over around 4 watt they need cooling so the 16/8th inch is reduced to allow room for the cooling fins, which mean less fire barrier, and also less lit area, so a ceiling with 6 x 50 watt needs around 10 x 3 watt to get same light spread, in the main my rooms are lit by reflecting light off the ceiling giving less shadows and a better spread of lights, moving from MR16 it may be better to look at larger flush fitting lights.
