The typical servile term for a master in Latin was dominus (masc) and domina (fem). This was so ubiquitous that dominus (or the direct address, domine) becomes the default address for a Christian to God. Even if the slave did not speak Latin, s/he would learn dominus on the first day. Children would also be called by the same.

Now, a Briton slave would not be "hired" for the house in the Imperial period. The slave would be bought, and would receive no pay whatsoever. Slaves in the Roman Empire were not like the downstairs staff in Downton Abbey: they received no pay, had no rights, and no terms of service.