I've not read anything written about this one way or the other, but it would be really random if it was written as acts. Watching US dramas on the BBC is always disruptive because you can tell where the adbreaks should be and it breaks the flow of the programme quite badly. So it seems unlikely that they would make a decision to emulate that style - even if the most jarring aspect of missing adbreaks, the music, wouldn't be there.

Plus, you know, the writers just wouldn't be used to writing that way.