Dual British citizens can no longer use an ETA to enter the UK. They must present a valid British passport or Certificate of Entitlement. If your British passport has expired, you can renew it at gov.uk. Airlines may accept an expired British passport issued after 1989 at their discretion.
Apply for an ETA at gov.uk/eta or download the UK ETA app. If you have settled or pre-settled status, you do NOT need an ETA.