Yep, thats british and live....... Dont pull the pin out. The 'M' denotes waterproofed. The bottom plug should unscrew, exposing the detonator, which can be removed but beware of picric acid crystals which can be very unstable.
Options are, chuck in a lake, pull the pin and chuck in a lake, unscrew the base if you feeling brave..... or give it to the local expo. The police will be able to contact them.
Nice find though, I have some lovely deac ones. Something like that would go for £50 at Beltring.