Time to command in BA is determined by seniority alone.
Currently the most junior commands are at LGW (737 becoming Airbus) followed by the Airbus at LHR. LH commands are the most senior with the 777 being perhaps marginally more so than the 747 at present.
Which route you take to command depends on you to a large extent.
If say a LHR airbus command would take 10 years, it makes no difference if you say spent 10 yrs RHS 747 or 5 years RHS LHR Airbus followed by 5 years LHS LGW Airbus. Type changes are usually subject to a 4 yr freeze.
The difference is lifestyle.
Regards