It seems pretty certain that you will have to address the matter by presenting your UK passport, or at least its details which will allow some government agencies to look up and verify them. Alternatively you could ask the Home Office to confirm your right to remain in the UK (giving them the passport details) and forward that confirmation - but they just possibly may be reluctant to confirm or deny, in the well known UK fashion where so little is absolutely clear. If in your passport you are stated as a UK citizen then you should automatically be entitled to reside in the UK, although what passes for a system in the UK sometimes results in permanent rights "expiring" or disappearing somehow (some of the time the authorities themselves don't know, or are in some sort of catatonic state over things). Leave any thoughts of complaining until you arrive at a clear final position and then evaluate whether the authorities have acted shambolically (which may well be the case).