"Existing halogen headlamp units on vehicles first used on or after 1 April 1986 must not be converted to be used with high intensity discharge (HID) or light emitting diode (LED) bulbs. If such a conversion has been done, you must fail the headlamp for light source and lamp not compatible."
In my opinion, this appears inconsistent if the LED headlamp passes the dipped beam pattern tests - however it is the regulation.
Advances with LED Headlight bulbs small enough to fit in the housing may mean a conversion to LED has become viable (August 2019).
This pair, Startway H4 LED Bulb , arrived in a stout box. Price not exorbitant (£35) and above all, clever design means they can fit into the 1007 headlight shells.
The wire spring is strong (see above) and these bulbs do have a thicker base so you will need to push even harder to engage/disengage the spring under the catch. I had two (of 6) that I could not latch and was forced to weaken the spring slightly. They can be removed from the light housing and weakened a little by making the “vee” a less acute angle.
This bulb saves space by having the fan inside the headlight housing.
This means there is no fan or heat sink to get in the way of the wire clips fitted to hold the bulb in place.
and the bulb fits snugly down into the lamp housing.
I took the car down to the garage and checked on the beam-setter. Beams look OK and the tester has so far given all three of our 1007's a pass without comment. The cut-off is not quite as good as the filament H4's.
On the road — the beams haven't upset anyone so far, and visibility on country roads is much better on winter evenings. After several 100-mile trips in the dark, on dip they pick out the white line on the side of the road much better than the Osram Night Breakers I had previously.
A post-COVID-19 trip (August 2021) of 100 miles in the dark demonstrated the danger of being forced to go back to Osram Night-breaker H4's as the oncoming traffic (mostly appearing to use white LED's as the ones still on filament bulbs were very yellow in comparison) caused me visibilty problems on several occasions.
I would also point out that, whilst the MOT station may well set up the headlights accurately, it is a matter of a few moments to tweak the aim slightly upwards - as a number clearly had. The point being that, however compliant at the MOT, subsequent adjustment to can invalidate this adjustment, but cannot be held to be illegal. So why is a good LED setup (that passes the MOT beam tester) illegal? This appears to be an inconsistent treatment of a safety issue.
LED bulbs sadly removed August 2021 to comply with regulations
I have a set of Philips Ultinon LED bulbs with similar specification to the Startway's. These have a heat sink on the outside which unscrews to allow the wire clip to retain the bulb. Not tried them yet so can't comment on how well they work.