Part of the reason that the UK LoM only lasted two seasons is that John Simm quit - but if they'd had the (originally intended) third season, there would never have been Ashes To Ashes, so it worked out better this way! Five seasons of the Gene Genie instead of three.
The endings...
The UK ending is way more emotional, and the US ending makes perfect logical sense in a totally cold and uninvolving way! Which makes the UK ending better - and the US ending pretty much precludes doing A2A.
FWIW though, the original scripted UK ending was going to be that Sam recovers, goes into a pub and goes all Gene on some thugs, having learned from his dream that that's how things *should* be done... Glad they changed it!
I think they should have kept an element of that though, like Sam does rough up some hooligans and then gets reprimanded for it, and that along with everyone else's complete misunderstanding of good and evil coupled with his disconnect of the real world contributes to him doing what he does. As it stands, the end of LoM while I loved it, seemed a bit too quick to pull the whole "the future is shit and every one hates you" card.
in the UK we had two shows end the same week with a similar ending. First "Ashes To Ashes" followed shortly after by "Lost".
And boy, did that piss me right off. 48 hours after the end of A2A and LOST pulls the exact same thing. Didn't help that LOST threw away a season of plot development for the sake of that ending too.