How Eric Brown combined private test flying with Royal Navy service

Could an Empire test pilot work for aircraft companies abroad while still serving?

Yes. Brown was loaned to Western and American companies for test work while remaining in the Royal Navy and eventually becoming a captain.

Brown undertook private-company test work and flew for American firms while remaining a naval officer. Such assignments were arranged as loans rather than a departure from service. Brown later became a Royal Navy captain. His route to higher rank narrowed when the service began reducing its carrier force, the branch in which his experience was most relevant.