I would not lie to immigration. I would explain that I renewed my passport in Malta during my holiday (usually just a week or so) and that I will be applying to renew the CoE-RoA in UK.

Thank you so much for your swift reply. So, to be clear, I can renew my Maltese passport as usual in Malta during one of my short visits there. Return to UK with both my old passport (with CoE-RoA) and my new passport. Then apply to renew my CoE-RoA in UK, and explain this to the immigration officer...

Question - I am EU/Commonwealth citizen (Malta) with Right of Abode since late 1970s. I do not have British Citizenship as I have never felt the need for it. I have a Certificate of Entitlement to the Right of Abode on my current passport. The certificate has to be renewed (used to be cheap but now ...