In the UK in 2026, the cost for extra baggage varies wildly between "low-cost" and "legacy" carriers. For budget airlines like Ryanair or easyJet, adding a 20kg checked bag at the airport can cost as much as £50 to £70, which is often more than the flight ticket itself. However, if you "pre-book" the bag online at least 24 hours in advance, the price usually drops to £25 to £40. For legacy carriers like British Airways, an extra bag typically costs around £60 to £75 for a one-way trip, though "Executive Club" members often get an extra allowance for free. If your bag is "overweight" (exceeding 23kg but under 32kg), you will likely be charged a "Heavy Bag" fee of around £65 per piece. The "pro-tip" for 2026 is to always use the airline's mobile app to add luggage before you reach the terminal, as "at-the-counter" prices are designed to be punitive to encourage faster boarding and more predictable weight and balance for the aircraft.