hello Linnylane, and welcome all the way from the UK, and sorry that your expectations to move back to Aus. hasn't really benefited you.
You have a long r/ship with your partner which is lovely but to hear that he is very unhappy concerns us.
If you did move back to the UK then his mental health, ( if I allowed to call it
that) there is no chance that he will improve, because feeling miserable doesn't disappear all of a sudden, and whether he is suffering from BP a doctor needs to diagnose him, and because he doesn't want to go, then I would be saying to him that you both need to have a checkup coming from the UK.
You can go before this because it's affecting you, and now you are suffering from some sort of depression, I would tell the doctor about your partner, but I'm sure that when he sees them it could be obvious, but the doctor would look after you first of all, because at the moment you're not strong enough to handle this situation.
We can't diagnose as we're not qualified, however if his mother has BP and his g/mother has mental issues, then he may also have it, so maybe getting another job may ease his problems but only temporary.
I'm 62 and know what it feels like to be in your position and hope to hear back from you. Geoff. x