I think that this is great! With UAL switching the 777's to the Pacific, it was bound to happen. UAL's only -100's left are used for West Coast-HNL and intra-Asia routes, but even then, we have just a few of them. The -200's are the most likely candidate to fly to LHR, but with UAL putting the new seat-beds (like BA's) on the -400, UA might switch the -400's to compete even heavier with BA.
*******CX******* what was your source?