My experience is BAH are non budging on dropping the EU leg in any way. They'll offer -3 / +14. Unless literally the day or day before cancellation.

My only options were moving to a different EU city within 300 miles. But the trip could not be changed to a UK starting point. I have however in the past on an exINV trip been able to drop the INV with a flight cancellation. It's all to do with EURO billing