It does not matter even credit check it’s done and tenancy started but even with credit check report it remind landlord/landlady to get a copy of passport.
Please try to remember that these are government rules so you have to give proof of ID.
Please note that you can also check the requirements by yourself -how to rent guide on gov.uk website
I always say to the tenant even I have done the basic check and even the person is born and bred and British I just say it you need to provide the copy.
Also I got a check list for all documents checked and given to tenants and signed before I give a set of keys tenants after all signed.
But please note if the landlady have give you property to manage to agents then tenancy has signed that in that case landlord/landlady can ask the proof of ID afterwards.
Also you have to remember that owner does not know you either but renting his/her property to you as well as.