nope, perfectly legal. There's something that states the flash rate, in the highway code, think it's something between 60 and 240 flashes per minute.
IMHO they're good as it's obvious you're a cyclist, and presumably the batteries last longer.
An advantage of front flashers is that in built up (well-lit) areas at night, you can always tell your front light's still working 'cos you can see the flashing reflecting off signs, cars' bodywork etc, which you can't always if you're on constant as the light could be coming from anywhere.