Well, Cadaha has basically got it in a nutshell, but (no offense), it is important to Quote LASORS in full. There is some detail to the pre-entry requirements which should not be overlooked.
For example, don't forget you
also need 5 hours (PIC or Dual) in a SEP aircraft during the 6 months Prior to the Pre-Entry Flight test. This is easily missed if you have just completed (for example) a Multi-Engine Instrument Rating. I would also emphasise that you
Must have satisfied the theoretical knowledge requirements at CPL (or ATPL) level
even though you do not require a Commercial Licence to commence the course.
Here is the text in full from
LASORS 2005 regarding the pre-entry requirements:
In order to receive payment for flight instruction, an
instructor must hold a valid professional pilots licence
and valid JAR-FCL Class 1 Medical Certificate.
Instructors who hold valid private pilot licences and
valid JAR-FCL Class 2 Medical Certificate are entitled
to instruct but cannot receive payment for this service*.
* This does not apply to the holder of a UK PPL(H) who
qualified prior to 1 January 2000 under national
arrangements.
Minimum Age
An applicant for a Flight Instructor Rating shall be at
least 18 years of age.
Validities
All instructor ratings issued in accordance with
JAR-FCL are valid for three years from the date of
passing the Skill Test.
H1.2 FI(A) RATING
Pre-requisite flight and theoretical
requirements
Before commencing a Flight Instructors Course (FIC)
an applicant must satisfy the course pre-entry
requirements as per JAR-FCL 1.335: -
a) Hold either a valid UK CAA or JAR-FCL aeroplane
pilots licence, which includes a valid Single Engine
Piston (Land) Class or single pilot, Single Engine
Type Rating.
b) Meet the knowledge requirements for the grant of
a JAR-FCL Commercial Pilot Licence
(Aeroplanes) as per AMC FCL 1.470(b).
c) Have completed at least 200 hours of flight time on
aeroplanes of which a:
i) PPL(A) holder shall have at least 150 hours as
PIC or
ii) CPL/ATPL(A) holder shall have at least 100
hours as PIC.
d) PPL/CPL/ATPL(A) holder shall have completed:
i) At least 30 hours on single engine piston
powered aeroplanes of which at least 5 hours
shall be in the six months preceding the
pre-entry flight test.
ii) At least 10 hours instrument flight instruction
in aeroplanes of which not more than 5 hours
may be instrument ground time in a FNPT or
a flight simulator.
iii) At least 20 hours of cross-country flight as PIC
of aeroplanes, including a cross-country flight
of at least 540km (300 nm) in the course of
which full-stop landings at two aerodromes
different from the aerodromes of departure
shall be made.
iv) Pass a specific pre-entry flight test with a FI
qualified as per JAR-FCL 1.330(f) based upon
the proficiency check as per JAR-FCL
1.240(b), within the six months preceding
the start of the course. *
*The pre-entry flight test will assess the ability of the
applicant to undertake the course.