The following is an extract of UK Athletics Rules for Competition:
RULE 9 COUNTY, DISTRICT AND REGIONAL QUALIFICATIONS
1. English County Qualification
(1) County Championships are open to all eligible athletes possessing a
County qualification.
(2) A County qualification may be acquired as follows:
(a) By birth
(b) By nine month’s continuous bona fide residence in the County
immediately prior to the competition.
(c) By service in a Unit of HM Forces stationed in the County for nine
months immediately prior to the date of the competition. Temporary
absence on duty shall not break a qualification acquired under this
paragraph.
NOTE 2: Membership of a Club affiliated to a County Association does not, of
itself, provide an athlete with a County qualification unless at least one of (a) –
(c) above is satisfied.
(3) Competitors who have qualified and competed, under (2) b or (2) c above
retain that qualification until they have acquired a residential qualification and
competed in the Championship of another County, or represented another
County in an Inter-County Championship.
(4) A student at school, college or university does not acquire a residential
qualification by residence in the County during term
time only.
(5) An athlete who has competed in a County Championship
or represented a County in an Inter-County
competition, may not compete in the Championships of, nor represent, another
County in that Competition Year.