The EEA family permit is defined in
The Immigration (European Economic Area) Regulations No. 1003 of 2006. See PART 2, EEA RIGHTS.
Right of admission to the United Kingdom
11.—(1) An EEA national must be admitted to the United Kingdom
if he produces on arrival a valid national identity card or passport
issued by an EEA State.
(2) A person who is not an EEA national must be admitted to the
United Kingdom if he is a family member of an EEA national, a family
member who has retained the right of residence or a person with
a permanent right of residence under regulation 15 [b]and produces on arrival [/b]-
(a) a valid passport; and
(b) an EEA family permit, a residence card or a permanent residence card.
(source: http://www.legislation.gov.uk/uksi/2006/1003/pdfs/uksi_20061003_en.pdf)
Then, the regulation defines what happens if you do not have an EEA family permit:
(4) Before an immigration officer refuses admission to the United Kingdom
to a person under this regulation because the person does not produce on
arrival a document mentioned in paragraph
(1) or (2), the immigration officer must give the person every reasonable
opportunity to obtain the document or have it brought to him within a reasonable
period of time or to prove by other means that he is—
(a) an EEA national;
(b) a family member of an EEA national with a right to accompany
that national or join him in the United Kingdom; or
And if you read further in article 12:
12.—(1) An entry clearance officer must issue an EEA family permit
to a person who applies for one if the person is a family member of
an EEA national and—
(a) the EEA national—
(i) is residing in the UK in accordance with these Regulations; or
(ii) will be travelling to the United Kingdom within six months of
the date of the application and will be an EEA national residing
in the United Kingdom in accordance with these Regulations on
arrival in the United Kingdom; and
(b) the family member will be accompanying the EEA national to the
United Kingdom or joining him there and—
(i) is lawfully resident in an EEA State; or
(ii) would meet the requirements in the immigration rules (other
than those relating to entry clearance) for leave to enter the
United Kingdom as the family member of the EEA national or,
in the case of direct descendants or dependent direct relatives
in the ascending line of his spouse or his civil partner, as the
family member of his spouse or his civil partner, were the EEA
national or the spouse or civil partner a person present and settled
in the United Kingdom.
(source: http://www.legislation.gov.uk/uksi/2006/1003/pdfs/uksi_20061003_en.pdf)
Not having an EEA permit before you head to the UK - you might not get a boarding pass when entering on board of your carrier, and you could experience major delays without one. Depending on the setting at the border, you may be refused entry into the UK if you don’t have an EEA family permit. An EEA family permit makes it easier and quicker to enter the UK. It is free of charge, so considering the fact that you are eligible for one, I would not hesitate to apply. Your partner needs to travel with you. The code defining the EEA family permit does not reflect purpose of travel.