The short answer
Poor ratings can affect running costs, lender appetite, and compliance with future regulations.
A low rating isn't just a higher energy bill; it can also narrow lender appetite and expose you to future compliance costs as energy-efficiency rules tighten. Worth factoring into the real cost of a property, not just the asking price.
Why this matters for your purchase
This is the kind of thing a PropM8 Property Intelligence Report is built to surface early, before solicitors, surveyors or lenders are involved, while it still costs nothing to walk away.