You are allowed to work on your EEA family permit as soon as you enter the UK. You actually are allowed to work as long as your spouse is exercising treaty rights here so you are free to work at present. The issue is that you can't prove it to an employer. Therefore, some employers will not be willing to take you on because you don't have evidence (in terms of a RC or other visa [once your EEA family permit runs out]) of this right.
If you know the employer well, you might want to show them the law (not sure exactly where you can find the part about being able to work) and hope that he/she is satisfied with that until you get your RC.
Here are some links that might start you off:
1)
http://ec.europa.eu/social/main.jsp?catId=463&langId=en (from the European Commission website: "Family members: do not need a work permit to work, even if they are non-EU nationals.")
2)
http://www.ukba.homeoffice.gov.uk/eucit ... ts-family/ (from the UK Border Agency website: "You do not need to obtain documents confirming your right of residence in the UK if you are a family member of an EEA national. However, you may be inconvenienced if you do not obtain this confirmation, as: -you may have difficulty proving that you are lawfully resident in the UK; -you may find it difficult to obtain or change employment.").
3) ttp://
www.newlandchase.com/nc_blog/article/a_ ... e_proof_of (from an immigration law firm: re: a case proving that non-EEA national family members of EEA citizens do not need to provide any specific paperwork to prove their rights to work as this right exists from the point that they enter the UK: "the Judge held that as the non-EEA family member of an EEA national there was no requirement for a non-EEA national to have any paperwork proving her entitlement to work in the UK. The argument was that she was automatically entitled to work anyway and the paperwork merely confirmed this."
4)
http://www3.imperial.ac.uk/hr/procedure ... permission (from the Imperial College London: "You have a right to work in the UK if you are: -a family member of a British citizen, EEA or Swiss national. You will normally be entitled to join them in the UK and look for work, regardless of your own nationality.")