Did Costner's awful half-assed attempt at an English accent irritate you guys as much as it did me?

Yes.

I think that Brits have a better chance of doing a US accent than the other way around for a few reasons;

Britain is a small place. If the actor is doing a regional accent then chances are that most Brits will have heard the original at some point, and will be able to hear blatent flaws. The US on the other hand is a huge place with many more accent variations. It's going to be easier to find a middle of the road US accent that cannot be regionally pin-pointed. Obviously if the role calls for a specific accent then this doesn't apply, but I would guess that the majority of roles do not.

Many of the 'upper class' UK accents are often quietly spoken, sometimes with a very stiff upper lip, eg 'Sandhurst' (Army Officers). This isn't second nature to most USians who tend to speak louder in everyday life, and the stiff upper lip thing is just unnatural. I suppose that Botox might help

There is more US content on British television and movie screens than the other way around. Admittedly many of the accents there are probably CA or NY, but Brits are exposed to more US accents in the media.

That all being said, I couldn't do a US accent if my life depended on it.