Upvote:2
You're a US national with a French multi-entry working visa. You entered France in September, and you have presumably submitted your documentation to OFII already. You are currently awaiting the 'attestation de reception' from OFII, at which time you will get your appointment and your vignette.
You want to leave France in and return in November, about 1.5 months after your initial entry.
As far I have been told by OFII, your visa alone (without the attestation) will let you pass at ports of entry for the first 3 months, they do not expect the attestation at the border, nor was I ever asked for it. You will have to convince the UK that you also intend to return to France, but with the combination of your nationality and your visa, I don't believe this will be too difficult.
I entered France in October 2015 (US passport holder, D-visa MULTI). I submitted my documentation to OFII upon arrival. I then left the EU for work reasons in December, returning just a few days after Christmas 2015. In that time, the attestation de reception (with the appointment date) was mailed to my home in France, and my appointment passed. I was let back into France with no problems, showing only my visa.
Even having missed the appointment, I was told to simply wait for another one to be mailed to me. I didn't end up having an appointment until Feburary 2016.
In your case, I do not believe that you will be required to show the attestation at the border, nor do I believe you will have any issues entering in September and going to the UK for 2 weeks in November. As a US national, you face slightly less scrutiny than nationals of countries who require a visa to visit. Even so, I believe that you and these people could both leave France and re-enter under these circumstances.
Simply put, I do not believe that the border control is 'synced' with OFII in any meaningful way, so they would have no way of knowing whether your appointment has been taken or not. Some prefectures take longer than others (some taking months). As long as you are within the 3 month window and have submitted your docs to OFII already, I believe you will be fine.
If you want to be especially doubly careful, you could keep a receipt of your mailed documents (recommended letter, for example), but I do not believe this would ever be asked.
Source: CampusFrance
During the three months following your arrival in France, you are authorised to travel outside of France and to leave the Schengen area without an OFII sticker. After that, the OFII sticker, or the OFII attestation of receipt in the absence of the fomer, is mandatory for travelling outside of France.
tl;dr With an entry date of 10 September, you are free to travel in and out of France and Schengen until 10 December.