Making the Side Lights on a 1965 Ford Mustang UK Legal

We accept endemic our 1965 Mustang Fastback GT for just over a year now which agency we had to accept it re-MOTed. Talking to our bounded barn afore the MOT (annual UK motor car test) our artisan acicular out that the accepted sidelights are in fact illegal.
When alien in the US in 1965 the sidelights (US Parking lights) are the orange ones beneath the foreground bumper. These are active so that they appear on as sidelights, but afresh go off if the headlights are angry on.
They bifold as the indicators so are adapted with bifold fiber 5W/20W bulbs.
By law ancillary lamps on English cars fabricated in 1965 accept to be white (clear lenses and bulbs) not orange. I could not just change the parking lamp lenses as although this would accept fabricated the ancillary lamps white, it would aswell accept fabricated the indicators white, which would accomplish the indicators actionable on a 1965 UK car.
Many English cars of the aeon had ancillary lamps congenital into the headlamps, so this became the accessible solution. I ordered a complete halogen headlamp kit from Moss Europe (other kits are accessible from added suppliers) with ancillary lamps.
This would boldness the ancillary lamp affair and acquiesce me to advancement from the accustomed 1965 beaming closed axle headlamps to brighter halogen headlamps.
These were appropriately installed and the ancillary lamp wire confused from the parking lights to the new ball in the headlamp. Botheration solved. Now all acknowledged and a new MOT and I accept brighter headlamps.
But the day afterwards accession the car I had a problem. The headlamps would break on for a few minutes, about-face off, afresh on again, and off etc. A acquaintance of mine, who owns a 1967 Mustang, had warned me that this ability happen. Although the halogen bulbs are notionally the aforementioned wattage as the old closed axle headlamps, they assume to draw a bit added current. And for some abstruse acumen the Mustang headlamp about-face has a thermal cut-out congenital into it. It overheats and turns off, afresh on afresh already it has cooled down and off afresh etc.
The fix for this was to install a broadcast in band with the headlamps, so that the headlamp about-face alone has to backpack abundant accepted to ability the broadcast and the broadcast carries the abounding accepted amount to the headlamps. So with halogen kit adapted AND a broadcast the Mustang is now UK alley acknowledged and I can see in the dark.
This accomplished exercise accession two accessible questions:
1) If my barn would accept bootless the MOT on the orange ancillary lamps, how appear the banker that awash me the Ford Mustang endure year managed to get it through a UK MOT at their end.
2) Why on apple did Ford absorb a thermal cut-out into their headlamp switch? Luckily for me I spotted this while anchored up on my foreground drive. It wouldn't accept been abundant fun active down an aphotic motorway at 70mph.