Lie or lay

I learnt in school that the verb “lie” is only used as an intransitive verb while “lay” can be either depending upon the context.

I clearly understand we should say “someone lies down” not “someone lay down” in the sense of someone laying themselves down horizontally in the bed or on the floor. Having lived in Australia for about 40 years I do hear people often say “someone lays down” in this sense though I wouldn’t copy. We could say “someone lays the baby down but we’re not supposed to say “someone lay down in bed” unless it is in past tense. Of course he can lay himself down in the present tense.

The other night I watched a British TV series Doctor Martin I heard the doctor say “Lie him!” and “Lay flat!”. I could understand if the latter was used to ask a third party to lay someone else flat but I wasn’t sure though.

Could someone comment on these please?