It will count as one piece of your luggage but if it is within the maximum dimension (190 cm in length is the key number in this context) it will not incur any extra charge as long as you can stay within your baggage allowances.

If the packed length (of the outer dimension of your packaged item and not just the actual rod) is 6'5", then you could be "in trouble" so to speak, as you can't check it in as part of your luggage because it exceeds 190 cm. If the packaged length exceeds 190 cm, it will have to be shipped as cargo (separate charges will apply).

It also must be packed appropriately securely for it to be accepted. You probably should check with your travel insurance to see if it is covered for loss or damage also, as airlines may not accept liability for such items (check with BA as to if they do, if you are interested in checking).