Heathrow is actually a significant hub for other than BA, as there are a substantial number of route operated by Star Alliance carriers who interchange there, with each others' codeshares on flights, and are progressively building up a nice single-terminal connecting facility at the new T2.
Unfortunately for them, their big loss was the end of BMI, which offered the connections to other UK trunk destinations, and to those European points which did not have other incoming Star Alliance flights. However, it was Star Alliance's own arrangements that were a significant part of the BMI downfall, as under established interlining division of revenue, BMI having the short part of say a LAX-LHR-DUB from United, or JNB-LHR-EDI from South African AW, through ticket, and just got a pittance for carrying the connecting passengers. Lufthansa, their principal backers, just thought such passengers should connect through Frankfurt instead.