Deliveries from Canada to UK used to be done in 15-20 days, in the 1850s, as confirmed by a few Montreal - London correspondance I have obtained. 160 yrs later, it appears that it has not improved much. COVID19 likely plays a part in current delays, or perhaps
Canada Post has shifted its priorities to parcels, and what used to be called 1st class mail (private letters) appears to be considered last class now...
I have had many below par service experiences with CP in recent years, with little (if any) improvement or satisfactory explanations.