I have an expired British Passport, which technically still allow me to "return" to the UK without applying for an ETA. I had done so in the past (before the ETA was a thing), used my Canadian passport to board the plane, upon arrival, border force would just scan both passports to make sure they are real and "valid" (As in I have not renounced it), without stamping on my Canadian passport.

Now I'm not so sure if I can get past the airport dragons without a proof valid ETA on my Canadian passport. I'm not going to take the chance.

Trying to save 16 quid by spending £100+ to renew my passport seems counter intuitive (especially after Brexit...)