Visiting the UK before EEA Family Permit is Valid

[ClaudM002]

Hello there, My name is Claudio, and I have a somewhat urgent question regarding the topic above.

Long story short, my wife lives in the UK and I've had plans to visit her to stay through to spend Christmas together. I purchased a ticket to leave on Dec 1 (after being informed of approval via email, but before physically recieving the EEA FP), but my EEA FP is valid FROM 29/12/17.

My question is whether anyone would happen to know if I am absolutely required to leave the country simply to return again, or if I can remain within the country. I understand that the permit has to be stamped (I'm assuming this as my Marriage Visitor Visa was stamped upon arrival)?

Also, after some research into this, I was given the idea to call customs of the airport within which I'd be arriving (Manchester Intl), and ask directly what my option would be. Unfortunately, my EEA FP is not showing up in their system as of yet. I'm assuming due to it being approved as of yesterday.

Any help at all would be greatly appreciated as this would be my first Christmas spent with my wife. If there are any alternative routes I can take to ensure that I can stay within the country, or, anything more convenient than returning all the way to the USA would also be immensly appreciated.

Thank you in advance for taking the time to read this!