A possible silly mistake I made at my biometric appointment for citizenship

Today I went for my biometric appointment for citizenship at TLScontact Liverpool and brought both my old and current passports in the same bag, because my old passport still has my previous visa in it (although it wasn’t actually necessary to bring it since I had already scanned and uploaded it by myself). When I handed my documents to the staff at front desk , I believe I gave them my current passport. However, when I went to do the fingerprint scanning on the self-service biometric machine (with staff assisting nearby), I’m not sure whether I might have used my expired old passport for the scan. If I did use the old passport, will it cause any problems?

Most likely it won't. Should it be a problem you will be contacted, they won't reject your application over this kind of mistake.

Even an expired passport is still a document that can be used as ID - if it's cancelled through hole punching, rubber stamping, defacing etc then it can't be used for anything.