Wow. Great info all, thanks.
As it's pretty much a solo work trip, I won't really be able to take much time either end. As several have pointed out, this makes an EU reposition a bit less attractive, and a bit more of a faff, if you have to get back to EDI.
I think I'll take the advice and look again immediately after Christmas for ex-LHR fares - I presume there will still be reasonable availability then...
If I book EDI-LHR separately but use BA, can I 'amalgamate' them later, so that for example I can book my bags through direct to EDI on my return?
Thank you all for your responses, what a fantastic resource. If I pull this one off I might get the bug - can't see myself desperate for another trip down the back of the plane!