Can you consistently say that a certain weed is better than another one?
Sure I can. I've been smoking the shit for over 20 years now, I know quality weed when I smoke it.

That said, I'm in no doubt that American weed will be, on average, better than British weed. Why? Because it's legal. Those who grow it here are, by the very nature of cannabis law, criminals. You're not going to get someone quit their job as a science teacher to grow weed. In USA, growers could be deeply intelligent experts who understand plant biology before getting involved with weed. People like mojo, rather than people like me. Also, investment is higher, better quality equipment, because there's no need to hide it from authorities.

That said, there's no reason why it's not possible to match American standards in the UK. It's just harder and more expensive.

I can argue this is evenidence of deregulation (in the form of decriminalisation) improving the quality of a product. Of course, you can counter that American weed is more regulated than British weed (license, quality control, permitted fertilisers etc) and that's why the quality is better. I don't think so, but I can't be sure. I can still find quality weed that is comparable to the best American weed, it's just harder to get hold of and more expensive. The obstacle to overcome is regulation in the form of law, that's what's making it rare and expensive.