Hi,
Unfortunately you cannot just buy a blank key for a Rover 75. It needs a motherboard and most important, a Microchip transponder that is matched to your VIN. You can buy a brand new one that comes already to start the car and will work the central locking via the door lock. The remote function will not work without a session on a T4 to match the rolling code. Rimmer Bros can help. Part number CWD000060 from memory.
https://rimmerbros.com/Item--i-CWD000060