ILR BRP impounded at Heathrow two and a half years after naturalisation

[thsths]

I would say consider yourself lucky. You no longer had ILR, and BRP was invalid. Trying to use it is inappropriate, and they could have made your entry into the UK difficult without your UK passport. According to your account, you were not inconvenienced in any significant way.

As a general rule, if you are citizen of a country, you should always use the respective passport to enter and leave that country. Many countries even require that by law.