Suggestions? Well, I don't think I've seen any legal lights that flash. They all seem to have a steady mode too, which means they should comply with the British Standard or the German Traffic Regulations or equivalent.
However, there's more chance of being struck by lightning than being fined for being legally unlit when using pretty much any flashing red tail light as long as it's not pulsing, strobing, morse-coding or other obvious illegallity - Norfolk police bicycles aren't legally lit either! Even the mycycle 5 LED light widely available in the pound shops will work fine as a town light.
Personally, I do have legal lights: dynamo with stand lights on the Dutch bike (Axa Pico 30 Steady front, Axa Riff Steady rear); Lidl 40 set on the road bike, plus various older setups which move between the other bikes and aren't on sale any more. All have red steady rears. I think flashing is harder to see and judge distance.