Jet2.com power-bank flight policy.

Short answer

Jet2.com is tracked from its official Dangerous Goods and Restricted Items page. Jet2.com states that lithium-ion batteries, lithium-metal batteries, and power banks without clear Wh or lithium-content markings, or where the rating cannot easily be ascertained, are forbidden. Spare/loose lithium batteries, including power banks, must be carried in hand luggage and placed under the seat when onboard. Customers may carry no more than 20 spare/loose lithium batteries up to 100 Wh / 2 g lithium content, no more than two spare/loose lithium batteries over 100 Wh and not exceeding 160 Wh, and no more than two power banks per person not exceeding 160 Wh. Power banks and spare batteries must be individually protected against short circuit, must not be charged onboard, should not be used in-flight, and must not be used to charge or power devices during taxi, takeoff, and landing.