One way for me. I am coming back with LX (regular one stop flight).
I made a dummy for you:
ABZ FRA LH973 on July 14th
FRA IAD MCO, LH 418 and UA 1471 on July 15th (4 hours at IAD)
MCO – IAD – FRA – ABZ (UA 1090 – LH419 – LH970) on July 31st and Aug 1st
Total: GBP 2100, “P” booking class (on Lufthansa.com, using the multi segments option)