Hi Sam-MAN, your question was specifically i.r.o. SELF-sponsored.......
As has already been said, you only really need GCSE-standard Maths & Science to get through the work. It's not rocket-science, but IS a HUGE volume in a short time! So being able to study and absorb a huge amount in a short time is a prerequisite!
HOWEVER, when the time comes for applying for a job/ Type Rating/ SSTR/ JOC, etc, you WILL need as high Academic qualifications as you can get as it can be VERY competitive. (look up various Schools' entry and/or assessment requirements, that'll give you a good idea)
So get the highest Exam Qual's you can & go for it!
BTW, make sure you can get a Class 1 Medical before you start spending silly amounts of dosh........................
Good luck. I know many who have done just that.