VS domestic
As to be expected:
Only LHR-1 and LHR-5 have domestic arrival facilities. There's plenty of room at LHR-1.
EI are in LHR-1 and they are operating the services.
It's unlikely that either BA or VS would want it to be at LHR-5!
LHR-5 has little spare space anyway as BA brings the remainder of its domestic operations from LHR-1 accross to LHR-5.
It's an easy transfer between LHR-1 and LHR-3 for pax, landside at least.