I used a 10% benzoyl peroxide acne cream. I'm from the UK originally, but am US based now. I just tried to change the address on Amazon to my dad's in the UK, but you're right, it won't show as in stock.
If you manage to get hold of some, just don't overdo the process. I was determined to make the stains vanish completely, but I really could have stopped when they were barely visible instead. The lightening really isn't that bad on mine and only in a couple of places, all below the brand line and all covered by the armour around the neck socket, but it can be avoided entirely. Should be a non-issue for you because your staining is all in areas in shadow, so you can just do the process until it's enough, rather than going overboard like I did!
, you don't need to add a lot (I added like 2-4 drops) and just let it soak in warm water for some time. I usually repeat this process 2-3 times for figures that have masks or skin touching cloaks, robes, and such. I'm scared of stains a lot! 
)
