How to prove 10 years residency based on records

My friend needs to transfer his ILR to a BRP. They are asking him to prove 10 years of residency.
Problem: He no longer has bills or other documents going back 10 years. He only has about 4 years.
So now the interesting part: How to prove you have lived in the UK for 10 years based on records? Most institutions only keep records for 6 years.

When living in the UK, naturally your data will be stored in various registers and records. But retreiving it is harder than you might think. What are possible sources? So far I have come up with:
GP - letter showing when you registered and you have attended for the past X years
Credit reference agencies - showing address history
Electoral roll archives

Any other suggestions?

Summit proof of his ILR and what ever else he has, etc. The burden of proof may be on them to show that it has lapsed, etc.