In summary, if you live in London EC, Manchester M, Birmingham B, or any rural area starting with LD or TR, your second-class mail is basically taking a scenic tour of the UK. First-class is generally fine, but don’t hold a baby shower for your parcel until after day three.
Remember: Royal Mail delays are temporary—unlike my eternal disappointment when I see a delivery driver pass my house and keep going. So check your postcode, laugh about it, and maybe invest in a carrier pigeon. They’re surprisingly reliable, but they charge by the biscuit.