In my book, therefore, it's courteous for a pedestrian to wear reflectives, but in no way required.
It's true that there's no law making them mandatory, but in practice you seem to get short shift if you play in traffic dressed all in black. For instance (police statement here):
"Whilst the driver of the Skoda recognised the hazard, she had insufficient time to react in order to avoid the collision.
"There is no evidence to suggest that either of the vehicles were being driven in anything other than the correct manner. The cause was the pedestrian walking along an unlit road, dressed in dark clothing, into the path of oncoming vehicles."
That was just the first hit that Google threw up, it's pointlessly depressing dredging for more.
On the principal that they didn't leave clear stopping space in front of their cars, should motorists who hit deer (or sheep) be prosecuted on the grounds that it could have been a pedestrian? Same mens rea in both cases, just lucky the deer wasn't actually a man in dark clothing I guess.