The most common reason for this kind of question is that the person wants to hold British citizenship without disclosing to the authorities of their other countries of citizenship that they hold British citizenship, perhaps because dual citizenship is not allowed, etc. So they want to continue to travel on a non-British passport and still be able to return to the UK.
As the OP's flag suggests that they are Brazilian but Brazil allows dual citizenship, either the OP is flying a flag of convenience or they are asking for a friend.
Two observations;
(a) As a matter of British law, once a person becomes a British citizen, any immigration status ceases immediately, on the day of the citizenship ceremony. It is possible that that status is not updated immediately on the government databases and that you may still get a share code. BUT, that will eventually stop, because that person has ceased to have an immigration status. So it is a matter of taking a gamble on how long the person can pull off traveling from and to the UK based on the share code.
(b) Such a person, now a British citizen, can
apply for a CoE-RoA in their non-British passport. That would allow them to travel to and from the UK on a non-British passport.
BUT
(i) the person can hold either a British passport OR a CoE-RoA in a non-British passport, but not both,
(ii) A CoE-RoA costs four times the cost of a British passport and must be renewed each time the foreign passport is renewed as well,
(iii) the person will not benefit from any of the visa-free benefits of having a British passport or rely on British consular assistance (consular assistance abroad will be determined by the foreign passport)
(iv) While having a CoE-RoA in a foreign passport is legal in the UK, it may contravene the laws of the other country/ies of nationality if they require the person to renounce their other nationalities on acquiring British citizenship. And the UK won't be able to provide consular assistance (see above) if the person is arrested or jailed in the other country of nationality because the person has broken their laws.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.