Can't find my Home Office reference number

Hello. I'm registering online for the Life in the UK test and it asks if you've a Home Office reference number, but I'm not sure whether I do or not. I only possess two letters from the Home Office and neither seem to plainly state a reference number. One letter was for my biometrics appointment when I applied for FLR and there's a case reference number, which I assume isn't the same as a Home Office reference number. The other letter was for my identity card and referred to my 'Unique Number', which is my biometric residence permit number. Would that be the same as my Home Office reference number? I'm worried that if I complete registration now and say that I've no Home Office reference number only to find out later that I do, that I'll have to take the test again.

Thanks in advance for your help.

Your HO reference number will be at the top of your visa approval letters. On my documents, it says "Our Ref.", begins with a letter and is followed by 7 numbers.

I began the visa process before biometric cards were required, so I don't know if it's the same as the number on your card, unfortunately.

It's not essential. I've had so many different visas and numbers all over them that I just never bothered putting it in most of the time. It's more of an issue for those on things like discretionary leave and asylum.