Thanks everyone for your answers. Looking at Oslo or Stockholm for my QR starting point and booking positioning flights with BA ex-LHR to fly 2 eligible sectors for status.
I’ll follow your advice and get there the night before. If BA cancels that one at least I could look at first flight of next day.
On the other hand is it okay to fly OSL-LHR on the same day? I’ve read QR are usually nice and check baggage through despite being separate tickets.