Can someone tell me what the author means when he says, "What it lacks in ultimate welly". What is "welly"? I'm an American and I'm not familar with that term. I looked up "welly" on the internet, but all I found was reference to Wellington boots.

Just guessing about the origins of the term, Tim, but it probably does come from the Wellington boot. To 'give it some welly' roughly means to add power, as in opening the throttle, turning it up to 11 or, indeed, giving it a kick up the arse (ass) with one's boot.

Don't they have asses in America, but we just have arseholes?