BC-Dual citizen exit UK on other passport and return to UK with ILR Vignette

Asking on behalf and seeking clarification.

BC applied for renewal of British passport, HMPO replied that on checking other passport there is a name not on it which is on the expired British passport and application renewal form e.g.
British Passport - ABCD
Other Passport - ABC
HMPO said to withdraw application, apply new other passport to reflect ABCD then apply for British passport renewal and send along new other country of nationality passport.
While application was made allowing for delays, the above outcome from HMPO was not envisaged and delays at HMPO longer than expected there is no time to apply for and get back other passport and then apply to renew expired British passport.
There is a more or less once in a lifetime family matter to be attended to and travel arrangements concluded.
Affected person can exit UK easily using other passport and use same to enter other country of nationality with no hassle.
He can also exit the other country showing to airline and immigration his ILR vignette sticker (no BRP card) stuck on expired passport of other country.
Can he enter back into UK by presenting ILR vignette sticker (no BRP card) stuck on expired passport of other country and valid passport of the other country of nationality?

PS: Do note that he has British passport already but expired but no time to go through procedure outlined by HMPO to get new British passport.

The ILR status is LOST when you attend your ceremony so it is not valid anymore. British citizens are not subject to immigration control so they cannot use a visa/ILR to enter the UK. The problem often is being able to show up at the UK border. The visa nationals must use a valid document in order to board a plane to the UK. When at the border, even a certificate of naturalisation can be used to enter the UK as a citizen

Zimba wrote: ↑

Tue Nov 23, 2021 11:35 am

The ILR status is LOST when you attend your ceremony so it is not valid anymore. British citizens are not subject to immigration control so they cannot use a visa/ILR to enter the UK. The problem often is being able to show up at the UK border. The visa nationals must use a valid document in order to board a plane to the UK. When at the border, even a certificate of naturalisation can be used to enter the UK as a citizen

Zimba, thank you on the clarification. He used his old ILR at other country's immigration posts and for boarding the flight, naturalisation certificate was presented at PoE to UK and accepted without delays.

Great