You will have to take and pass the JAR-FCL CPL examinations which now involves completing a 200 hour modular ground school course. The UK CPL exams are stillavailable but not for much longer. At the moment there are no JAR-FCL CPL theoretical knowledge courses available, but there should be in a few months. You will then need to complete a modular CPL course (25 hours flying) and pass the CPL Skill Test. Next you will have to complete a 30 hour FI Course with 128 hours of groundschool and passs the FI Test.
With the hours you already have, you could complete the FI course before the CPL course if you wanted, but either way you would have to pass the ground exams first. You can add the FI rating to your PPL but cannot be remunerated as a PPL holder.