Immigration status confirmation for mortgage application

Hello,
I am in the process of getting mortgage from Nationwide and have ILR. They have asked to supply the immigration share code, which I apparently can't generate as I haven't used the ID Check app and don't have UKVI account? Copy of my BRP or right to work/rent codes don't seem to satisfy them. Is there any other other way to confirm ILR status that I can't think of? Would Home Office issue a custom letter for this purpose if I request one?
Many thanks

On the nationwide website there’s a long list of proofs you can provide including BRP.
https://www.nationwide.co.uk/mortgages/ ... ationality
Perhaps ask them to check their own website.