Up until about 10 years ago, LHR was a "controlled" airport of some sorts the result of which meant CO had no access to LHR. That's why their flights all went to LGW. When LHR opened up, they quite naturally moved all of their operations from LGW to LHR (where most everyone wanted to go anyway). However, LHR, unlike LGW, was slot controlled and CO decided not to use one of those precious slots for a flight from CLE (other than for less than a year). That's the real reason they discontinued Cleveland - London service.
Hopefully, someone working at CLE knows the annual O/D traffic between Cleveland and London thought that may be giving them too much credit. Regardless, there's no natural carrier for the route. UA is the dominant carrier in CLE though it's no longer a hub. Star Alliance has no hub at LHR which is dominated by OneWorld (but is only
#2 or
#3 at CLE). If BA can't make service to PIT viable where OW (AA) dominates then why would they bother with CLE?