Firstly I have to say I enjoyed this film, and I suppose that's what counts. But it suffers from very low budget and it shows. Clearly most of the money went of the leading actors' salaries. Research and continuity were minimal. Portal's and Harris's medals are wrong. Ranks are wrong; Harris starts the War as an Air Chief Marchal, then for a few scenes is a mere Air Marshal and then an Air Chief Marshal again. Portal, his boss, is a Marshal of the RAF throughout the War although in reality he was only promoted to that rank after the War. It seems there was only one 1940s desk lamp in the props dept, as it appears on every desk including Harris's, even though it's well known he favoured the Anglepoise.
I also think the story of Harris and Bomber Command is too big to fit into 1h30m. Necessarily, huge amounts of the War are omitted... but we aren't really told, so one scene will transition to the next and only by picking up hints is it clear that we've jumped forward 3 years.
Good performances though. As I said, I enjoyed it, it kept me engaged, and I'd watch it again.