Immigrationboards.com

Once you’ve got your certificate of British citizenship, you must send your biometric residence permit back to the Home Office.
Send it back within 5 days of either:
going to your citizenship ceremony
getting your certificate of British citizenship

Hi there, I hope someone can help me with this problem am facing. I was unaware of the fact that I need to return my biometric withing 5 working days. I am travelling to Mauritius on the 19th of November with my whole family (they hold British passport already) and coming back around mid december. How would I be able to enter the uk without my biometric card and I am aware that applying for 'the right to abode' would take too much time.

would I have to cancel my holidays?

any help would be beneficial

You will be at high risk as your BRP will be void once you've attended your ceremony, in the same way that a bank card can be cancelled by the issuer.

The airline won't permit you to board without the relevant confirmation of your right to re-enter the UK, even if UK Border Control accept your British citizenship certificate.

I haven't seen any reports on the forum of the £1,000 fine being implemented, but that's not to say it hasn't happened yet, or when it will.

Mauritian citizens can enter the UK visa free as visitors...could OP just wing it and travel with Mauritius passport? Then at the UK border he can present himself as a British citizen with naturalisation certificate, have a minor interrogation, and eventually be let in as a citizen?

Yes indeed they could.