Britain has had it's fair share of metal bands. Iron Maiden, Judas Priest, Def Leppard, Motorhead, Black Sabbath, Deep Purple, Led Zeppelin, DragonForce, Asking Alexandria, Bring Me The Horizon.
Water-Ice is still weak to Electric, so Water-Ice and Fire-Electric would break the type triangle. That being said, it could still happen, Gamefreak has broken the type triangle once before, in gen 4, where we had Fire-Fighting and Water-Steel 3rd forms, Water being super effective against Fire and Fighting being super effective against steel.
Yeah so I looked into it and you're right we absolutely could see grass-dark. Apparently punk rock was huge in Britain and they popularized the actual punk culture aspect of punk rock. And metal, well damn, I didn't realize just how many legends of metal came from the UK. So I'm definitely starting to see where they could go with this.
And yeah, what I was getting at with water ice was precisely a deliberate reversal of the type triangle, so that there could be two type triangles, one going one way and one going the other. So your rival picks the type advantage against you at the beginning of the game only to find that surprise -- when they evolve, the type advantage reverses! Now you have the advantage against your rival! Of course if they want to do that, there's plenty of ways to do it.