To remove the FI restriction you as previously mentioned supervise 25 solo student flights,not hours, this means you sign them out as you are still supervised by an FI unrestricted, you also require 100 hours instructing, you then fill in the relevant form and send to gatwick, note the CAA like the 25 flights to be a broad spectrum of the PPL syllabus,the restriction is then lifted but does not let you teach IMC, this requires a 5 hour course and 35 hours groundschool by an FIC instructor plus a skill test, you must already hold an IMC rating.