It's a no brainer for me:

Transfer 50k Amex to BA and book Aer Lingus business class (just 25k each way, best transatlantic deal out there). Buy cheap ticket to Boston and look into tickets to Edinburgh/Glasgow with Aer Lingus OR Ryanair.

From experience, in Boston they will check your bag all the way to EDI/GLA if you're connecting with Aer Lingus. With either Ryanair or Aer Lingus, though, you will most likely have a long connection time in Dublin (annoying to wait, but chance of misconnect is low). However, my business class boarding pass from my previous flight has been enough on three occasions to let me into Aer Lingus' rather comfortable lounge.

Also, why not spend a day in Dublin? Easy city to see in a day, fly out to Edinburgh the next?