Are you wanting to take it as hold luggage, or cabin baggage.

Cabin baggage has strict size regulations and at LCY this is more strictly enforced as the aircraft are smaller. So it could well be a problem.

Checked hold luggage would be easier, though if the piece is exceptionally large you should call BA and check with them first. They will ask the dimensions and weight and then advise if you can take it. If it can be taken they will probably notify the airport to expect it and Load Control will need to be advised that the item has arrived. Remind the check-in agent to notify Load Conrol of this fact.

If it's too big for hold baggage it will need to be sent as cargo and BA can advise you how to do this.