Whilst 300 hours is a typical level for a PPL FI, the precise requirement is:
FCL.915.FI FI — Prerequisites
An applicant for an FI certificate shall:
(a) in the case of the FI(A) and FI(H):
(1) have received at least 10 hours of instrument flight instruction on the appropriate aircraft category, of which not more than 5 hours may be instrument ground time in an FSTD;
(2) have completed 20 hours of VFR cross-country flight on the appropriate aircraft category as PIC; and
(b) additionally, for the FI(A):
(1) hold at least a CPL(A); or
(2) hold at least a PPL(A) and have:
(i) met the requirements for CPL theoretical knowledge, except for an FI(A) providing training for the LAPL(A) only; and
(ii) completed at least 200 hours of flight time on aeroplanes or TMGs, of which 150 hours as PIC;
(3) have completed at least 30 hours on single-engine piston powered aeroplanes of which at least 5 hours shall have been completed during the 6 months preceding the pre-entry flight test set out in FCL.930.FI(a);
(4) have completed a VFR cross-country flight as PIC, including a flight of at least 540 km (300 NM) in the course of which full stop landings at 2 different aerodromes shall be made;