I have never pre-booked skis on TATL flights, but then again I've never flown BA on ski heavy routes like GVA or ZRH. As long as the measurements of your bag is less than
190 x 75 x 65cm and the weight of the bag is within your baggage allowance it's fine. You can try the chatbot at the bottom of the baggage assistance page
https://www.britishairways.com/conte...SeatingBaggage