Hi there,
I’m not sure about stock levels but since there is more than one Cartier boutique at Heathrow now I’m sure if the terminal you’re flying from doesn’t have your choice another one will. I think you can call ahead and reserve as well, either directly or through a personal shopper, but not sure if they can transfer items from one terminal to another so maybe good to check that in advance of your travel in case you need to plan extra time to transfer terminals with a personal shopper. I bought a Cartier watch a few years ago at Heathrow and they reserved it for me without a problem. I’ve also had a friend order her wedding bands to the Heathrow boutique to pick up on a flight out to get the duty free price! Unfortunately you don’t go through duty free after your return flight so you’ll need to pick up whatever you want on your way out-but I’m sure with some planning you’ll get what you want! Happy anniversary in advance!