As far as I understand it, technically according to the Highway Code or the LVLR Regs, which were written shortly after Humphry Davy invented the incandescent lamp and have not been updated since teh invention of LEDs, you're probably not supposed to use a flashing LED. Or at least, you might be allowed to (I'm not an expert) but it doesn't count as a suitable illumination as far as the law is concerned.
However, given that loads of people use flashing LEDs these days, and those that don't probably don't have any lights at all, I think it's extremely unlikely that you'll ever have any problems from anyone regarding this. You'd have to be pretty unlucky for someone to do you for having the wrong sort of lights. I always use flashing LEDs (possibly in conjunction with a serious halogen if really dark), but I don't have any reflectors (which technically would be an equallt heinous crime against society).
The only thing I would give any consideration to on this subject would be whether they enable you to see rather than just be seen. But if the roads you use are well-lit, it won't matter. (Some roads near me are not well lit at night, hence my halogen light which is powerful enough to ride a mountain bike through the woods at night, so I can see where I'm going, rather than just be noticeable to cars).