Welcome to Flyertalk and the BA board cmclaren, it's good to see you here, and I do hope you will have a chance to participate in the other threads in this and related forums.

I think you can plausibly argue that you were more than 3 hours late getting to EDI, given the re-routing, but you will need to point out that this is based on your booked end point rather than GLA. The amount is either 250€ (late shorthaul) or 300€ (late longhaul) depending on your point of view, or even 600€ if you argue you were more than 4 hours late. So have another go at it, write a short note back to BA, and be prepared to push for a better outcome.