The other lights have a sealed, non-removable battery and built-in charging. The P15 is designed to use a standard-size (18650) rechargeable battery, but neither it nor a charger are included.
The P15 is an older model, hence the low price. The 18650 battery is physically much larger, and holds much more energy than the built-in batteries of the other lights. Expect another £15-25 for a battery and charger, depending on where you get them. Note that batteries sold on Amazon and Ebay may be fraudulent and dangerous.