Sorry I use deep litter (and haven't experienced MD or MG). Hopefully someone in your climate (maybe check your state thread) has experience with sand and can give you feedback as to their experience with it.
OP specified: "My run is not sheltered from rain so I need something which can withstand water. (I live in the UK)" - so probably as gloomy and wet as WA lol.
I have only recommended sand to folks in dry, arid climates, or with well covered runs. Haven't tried it myself, no inclination to, but...
As long as drainage at the location is good, wood chips will continue to allow for drainage even if the run is not covered from rain.
The chickens won't really be able to "forage" on it but they can at least dig around in it.