I'm not against diversification so tell me what the negative of holding GBP vs dollar and why GBP vs another currency.

While I doubt the hyperinflation of the dollar happening, it would have global consequences. The largest GDP in the world with #2 and #3 as the largest foreign US debt holders. The US has the largest military budget. There's special relations with England is not part of the EU. While I could see the steady decline of the US in the world, a fast decline affects everyone.