UK providers (mobile, broadband, insurance, energy, gym memberships) typically offer better rates to retain customers asking about leaving than they advertise. Most customers never ask and pay loyalty premium for years.
How to do this
Compare market alternative prices first (uSwitch, MoneySavingExpert). Call provider's retention team (different from customer service). State you're considering switching unless they match the alternative. Most match or beat.
Annual exercise takes 2-3 hours of phone calls. Typically saves £200-500/year for an average household. Highest-ROI 'work' available to most adults.