You need good GCSEs (As and A * s), but honestly GCSEs are nowhere near as important as A Levels .
To become a surgeon (or any other sort of doctor) in the UK you *must* have A Level Chemistry, plus at least two, pref 3, other A Levels in Biology, Physics, Maths or Further Maths. A Level grades must really be A or A*, altho. you'll *maybe* be able to get away with one B (as long as it isn't for Chemistry).
Then it is:
1) 5 years of medical school to get your MB BS (the basic medical degree that all doctors have), then
2) 1 year working as a junior doctor (before you can register as a fully fledged doctor), then
3) at least 6 months working in either A & E or a surgical speciality, then
4) 3 years working in a Basic Surgical Training post, at the end of which you must
5) take and pass the MRCS* exams (*Member of the Royal College of Surgeons), then
6) at least 1 year in a senior training post, then
7) get your "Master of Surgery" degree, then
8) 6 years of specialist surgery training, then *finally*
9) take and pass your CCST (Certificate of Completion of Specialist Training).
So, altogether it takes at least 17 years to become a qualified surgeon in the UK, altho. unlike in the US you can start medical school at 18 yrs old here, straight out of school. A person would still be at least 35 years old by the time they were fully qualified as a surgeon.