His weird accent

To my Canadian ears that have somewhat adapted to British accents, I would say he was doing a Welsh accent (and my Scottish wife agrees).

Welsh

Welsh,
"I'll have another Bacardi and Coke please, Myfanwy."

Literally I watched this flick only to hear Bains' voice like as if I might hear it again like in The Dark Knight Rises. Just before I had watched Inception; didn't know Tom Hardy was in it, but instantly recognized that voice.

I was disappointed not to recapture that same unique tone.

It IS Welsh. Locke is based on Rob Brydons (Welsh actor) character in "Marion & Geoff".

I thought he was from South Africa (but had spent a substantial amount of time in China - enough to pick up a slight accent)

This is all Tom Hardy, period. True story. As authentic as his accent may sound to some viewers, the truth is plenty of people complain about the way he sounds in a number of movies. So, call it a Tom Hardy thing. :)

He’s got a cool voice.

He's just Welsh is all.