This isn't about countries with 15% corporate tax rates. This is about countries, like Ireland or Netherlands, with 0% corporate tax rates.

The UK loses a huge amount of corporate tax from EU tax havens, and the opposition has formed because the purpose of this deal is to allow the US to raise their corporate tax rate...rather than stop corporate tax avoidance (the US doesn't suffer hugely from this, all the profit in places like the UK that goes to Ireland ends up back in the US, and gets taxed).