MW2 was only an 18 because of no Russian, it even says in this BBFC article:
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Also Black Ops was only an 18 because of some slow motion moments in which you see the bullet go in slow motion and alot of blood come out, again this is proved by the BBFC:
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Cod 4 was fine as a 16+ there was nothing wrong with it.
WaW on the other hand should be an 18, think of the 1st scene, it is a torture scene and is very violent, also think of the bayonet charges!
And there is zombies, think of der riese there are tons of bad language (more than black ops zombies) and lost of violence.
I dont think MW3 should be an 18 by bbfc, there is never much violence in the modern warfare series. But as for PEGI they have rated all CODS except cod 4 an 18, but PEGI are way to harsh.
Thanks for the post and I hope mine helps you too