Is the 5K BIS miles? Or do the EQM Class of Service mlle count?
One way LHR-LAX (not the other way) in B is about $1500. Roundtrip LHR-LAX-LHR in B is about $1200. Go figure!
If you buy one way LAX-LHR (in M) for $1350, then one way LHR-LAX for $1200, you will have spent $2550. Prob cheaper than a roundtrip LAX-Hawaii in B!