From BA.com, very clear I would say.

If you would like the seat next to you to be kept free for additional personal space, you can buy it as an extra seat for comfort.

You will need to buy yourself an extra seat for comfort if:

• you're travelling in economy and need more space to easily fasten your seatbelt for take-off, landing or during periods of turbulence
  or
• you're unable to fully lower both armrests of your seat as it is a safety requirement to have a fully lowered armrest between each passenger during take-off, landing and turbulence.

We can provide you with an extension seatbelt, if you should need it.

Please note:

• An extra seat does not give additional seat width – see our seat dimensions guide on this page for the minimum seat widths on our aircraft.
• On most of our long haul aircraft, the armrests in the economy cabin do not fold away into the seat back completely. They stop at an angle of 45 degrees when lifted and cannot be raised further.
• There must be a fully lowered armrest between each passenger during take-off, landing and turbulence.
• Armrests are fixed in our First, business (Club World) and premium economy (World Traveller Plus) cabins and cannot be raised or lowered.
• You cannot buy an extra seat for comfort at the bulkhead or on an exit row.
• You will not be able to buy an extra seat on a British Airways operated flight if your journey includes a flight operated by another airline.