The plot, though on the surface doesn't run much deeper than a Wikipedia summary, is executed pretty well. Though repetitive at times, it stays engaging for the majority of the film.
In terms of the technical aspects, director Steven Knight does the best he can with the obviously limited settings, with creative lighting and interesting shots out of the window, but such limitations can only hold up for so long. Of course, such barriers cannot be overcome, but the lack of different set pieces does become tedious after a while.
Overall, Locke is an engaging film which becomes wildly elevated by the performance of Tom Hardy.
There's no human antagonist in here. The bad guy is Ivan's past, who is making consequences threading his personal and professional relationships. We don't know what's coming next, of course in the last shot we saw him driving ahead, probably to meet his new kid, but will he get his job back? Is he coming back home? Will his wife forgive him? That's the cliffhanger of this film.
As I mentioned before, there's no need of a big argument to make a good film out of it. A normal person's life is a different movie every single day.