I'm not convinced that watching fireworks from a distance is the best way to see them. It can be quite amusing to stand on Canonbie and watch the fireworks below you. But I think you miss out on some of the excitement of being a bit closer.

In the last couple of years there have been good displays from somewhere close to FH town centre, probably All Inn One. If they are having a display it might be worth going to that.

Blythe Hill does sound like a sensible compromise location as you should get a good view of Blackheath from there, as well as other displays.