They call and do an intense back ground check.
We use a priest.
If its not on the list you can't use them and they genuinely prefer someone irish.
They may ask for an affidavit ( did for my middles passport after my husband passed)
It's very strict, if not on the list they will send back.
Now any questions you have call the embassy in London. As he is your partner they will say no. But if you refer to family friend and clarify you can use.
If you have a parish priest always golden with them. I've an Irish friend who is a deputy head... and that was a firm no.
But honestly as strict as they are it depends who is doing your application.
Good luck I'm in the process for my youngest and I will need a solicitor again ( apparently they do not recognise a British death certificate) but like I say depends who you get on the phone.