As far as I'm aware, you can attempt the exams whenever you wish - but to commence an approved JAR CPL(A) Modular Course you need to have an ICAO PPL(A) with at least 150 hours flight time as pilot. But do bear in mind that you'll need to apply for the CPL(A) or IR(A) during the 36 months from the date of the final exam pass.......and that you must obtain an IR(A) during this period to keep your exam credit towards ATPL(A) - it'll remain valid for ATPL(A) issue for 7 years from the last validity date of the IR entered in your CPL - so be careful not to let your IR lapse for too long if the job market is still lean..........
UK summer - what's that...........??