I feel the most fair way to do it is as follows:
1. When the car is at the "hypothetical" stage, i.e. does not yet exist but we know it is coming - such as the 12C Spyder - the dealer should put any expressions of interest on a sheet of paper, the order of which is unimportant at this stage.
2. When the dealer decides to formally create its ordered waiting list, it should send a communication first to all existing McLaren customers, i.e. an email that goes out to all owners at the exact same time. Therefore, existing owners get first right of placing a deposit and establishing their rank on the ordered list, nothing more.
3. After a reasonable period, say one week, to allow for follow up phone calls (in case the email was missed), the list should be opened up to the rest of the market by first issuing another communication to all those who had previously expressed interest but are not existing customers.
4. Once the ordered list has formally started, nobody should be able to jump the queue.