Yes, flashing rear light on its own has indeed been legal since 2005.
I agree that a combination approach is probably best. It has been suggested that it is more difficult for a closing motorist to judge the distance to a flashing light. I run 3 rear lights on my bike, one steady, two flashing, to provide more visibility and some redundancy.
Interesting article here about hi-vis, including the question of whether it is actually the best solution in all conditions: