Seumas
Special permitted lens masks were (and are still, as far as I know) readily available in the UK, at least in motoring organisation shops, especially at the border such as the Channel Tunnel entrance. As you know, directional dipping was achieved by the lens.
More recent lenses do not lend themselves to such treatment, AFAIK, so it's useless when I take my newer RHD vehicle to the Continent.
I brought my LHD W123 from Germany to the UK. I drove it legally in the UK with a mask, until one day I smashed one glass, which needed to be replaced, so I replaced both lenses with RHD-compliant ones (even though the steering wheel was still on the left...).
(Slightly OT: Amusingly, when I sold the car 2 years later (still in UK) the buyer complained about the UK-compliant lenses...:-) He was a Portuguese who was planning to take the car to Portugal...)
I am also reminded of a later incident (in 1986) with a W124. I had taken delivery of a UK-spec car in Germany and managed to have an accident in Germany (snow and ice are my excuses), which needed new headlight lenses. I had expected LHD lenses as an interim measure, until I brought the car to the UK but, in the 48 h Merc Hamburg needed to fix the car completely (severe body damage at the front) they also supplied the correct RHD lenses.
When I mentioned Merc garages, I meant a main dealership. They should be able to source parts even for much older cars.
DAS