Agree with all your points S130. I've now performed some simple measurements on the Halfords led. I used my multimeter to measure the resistance across the terminals, and it read open circuit on all ranges, which suggests there is no load resistor across the input. I then measured the current at 12V, which was 0.36A, giving a power dissipation of 4.32W. The filament lamp came out at 21W, amazingly!
So, the led array has to get rid of, probably, about 5W under engine running conditions. Unfortunately I couldn't find my thermometer, but the metal case didn't get too warm after a couple of minutes, and as you said it is mildly finned. There appear to be 21 leds. If we assume they are arranged in groups of 3 in series, like the ones in the video, that would be 7 strings of leds each taking just over 50mA. This seems about right. Presumably a few hundred mA is enough that the bulb failure circuit doesn't trip, without the need for a resistor.
I wasn't prepared to take my lamp apart, but this seems to be a properly engineered unit. It's a shame that none of these lamps are type approved as in a P21W application they would be fine, provided they met the luminance requirement. I too am a retired engineer, please feel free to criticise the above!
Finally, you may well enjoy trawling Big Clives YouTube channel. I found his videos informative, amusing, and sometimes scary when you see what's available on line!