I'm pretty sure that the National Express coach takes about 10(!) hours. When faced with the same question for a September 2022 trip, I booked two refundable National Express tickets, one Edinburgh to Leeds, and the other for the next day Leeds to London, and a refundable hotel reservation in Leeds for the night in between. I ended up cancelling my whole UK trip for unrelated reasons, so I can't say how that would have worked out.

Granted, this was definitely not a graceful plan. I'm a budget traveller with a pretty long trip planned (and a preference not to sit on a bus for 10 hours in a row) so it was a decent option for me. Hopefully, you'll have better suitable options. But I just thought I'd throw that out there in case you don't.