Probably the starter but ....
It is probably the starter but in the odd case, a new battery cracks internally and hence does not put out but may still show 12 volts. Also the alternator could have quit and is not charging the battery up.
I would suggest that you take the battery somewhere and get it load tested. That tends to show if the battery has internal problems.
Also if it is the battery and not the starter, you should beable to jump start your 3 from another vehicle.
Note that one is supposed to attach the jumper ground cable to the body or frame etc of your 3 and not to the negative post of the Land Rover starting battery. I do not know why but apparently it matters. It may have something to do with sort of taking the battery out of the starting circuit, particularly if the 3's starting battery is defective.