Evidence of sponsor's immigration status in the UK

[wxyuanilr]

My wife is going to extend her spouse visa for another 2.5 years on the 5-year route. As her sponsor, I have ILR (Indefinite Leave to Remain) in the UK. In that case, should we just submit a photocopy of my BRP (Biometric Residence Permit) card as evidence of my ILR immigration status? Do I need to submit the original BRP card at all? Do I need to submit a copy of my passport if I'm not a UK citizen? I think a non-UK passport isn't able to serve as an evidence of UK immigration status, while a BRP card is. Is there any formal document from the Home Office explaining this? Thanks.

[seagul]

Maybe notarised copies by solicitor or certified copies by post office works. But I think letter come along with the news of ILRapproval may serve the purpose as well.