You need :-
200hrs total time of which 150hrs are P1 unless you hold a BCPL, CPL or ATPL in which case only 100hrs need to be P1.
to have passed CPL exams.
to have completed at least 30hrs on single engine piston powered aeroplanes, five of which should have been completed during the six months preceding the pre-entry flight test.
received at least 10hrs instrument flight instruction.
completed at least 20hrs of cross country flight as PIC.
passed a pre-entry flight test with a FI qualified to carry out such a test
Completion of all of the above allow you to start an approved course of training for the issue of a JAR-FCL FI(A) rating.
Full details are in AIC 105/1998 (White 325) which is available at
http://www/ais.org.uk/publications.htm
Hope this helps !