I don't know what an IGCSE is, but GCSEs are usually taken at the age of 16, which is the minimum age at which you can leave school in the UK.
A-Levels are taken at the age of 17 or 18 and represent the standard level of educational achievement in the UK these days.
The IB or International Baccalaureate is the Europe-wide standard, which is supposed to be similar to the A-Level.
However the Labour government in Britain has lowered the difficulty of the A-Level in recent years in order to improve the number of higher grades. They do this because it's easier to make the exams easier than to improve teaching or fund schools adequately.
[Political rant over]
