Restricted and prohibited items - British Airways

What the official page says

Aircraft seat power supplies are to be used exclusively for charging personal electronic devices (PEDs), not power banks. Power banks must be stored either in the seat pocket or in a bag placed under the seat in front. Power banks must not exceed 100Wh. Max. 4 spare batteries per person (of which up to 2 may be power banks), kept in original packaging or insulated/protected from contact with metal. On British Airways flights, you will need approval for each passenger carrying lithium rechargeable batteries of 100-160Wh or lithium metal batteries containing up to 8g of lithium metal. Spare batteries are forbidden in checked baggage. If your cabin baggage is checked in or removed at the gate and placed in the hold, you must remove all spare batteries and power banks and carry them with you in the cabin.