They have the ability to add a referral code post order, some have reported success particularly with the Edinburgh branch however Tesla's policy is clear that it must be used at the time of order.I ordered in the sheppards Bush store they didn't even ask if I had one. I wonder if you call up they can/will do something?
I suspect if you're to have any chance, speaking face to face with your sales advisor in the store would be your best bet.