The reason the older flashers would quit working when one bulb was out is the flasher needed the currant draw to heat the bimetal tang in the flasher, it would heat up and pull away from the contact then it would cool and make contact again. Electronic flashers don't care how many lamps are in the circut as long as you don't exceed the load rating. Thats why they work well if you pull a trailer with your car or truck. They have a clock that times the flash so the flash is constant to a very low voltage.
When you run the lights, or put on the brakes it reduces the available currant to the flasher unit so the flasher takes longer to heat, sometimes they don't get hot enough to flash at all. Make shure your charging system will make around 14 volts with the lights off. The newer three phase charging systems generaly take care of the low voltage and weak lights on the old british bikes.
Years ago when I was a wee lad, I worked in a shop that sold British motorcycles, and once or twice a year a guy from BSA/Triumph in the UK. would come to visit, he and the owner would go fishing and ride motorcycles in the hills. One evening after closeing we were having a beer in the shop, and I made the comment about the beer was cold because the fridge was made by GE not lucas, he had heard all the warm beer jokes. He said look the British bike industry was willing to pay 20 pounds, that was about $60 at the time for the entire electrcal system that encluded the chrome headlamp, that was in the late sixtys. He said that if they had spent just 5 pounds more they could have had the same quality as the Japenese had, but the bean counters said no as they were always trying to save a buck rather making the machines better. Things did improve some but by the time that started it was pretty much over.
Ken G.