Grade inflation. It's a huge issue. Durham University concluded from their research in 2008 that an 'A' grade at a-level today is equivalent to a 'C' in 1980s so you can truly understand the extent. Obtaining an A at A-level is now regarded an average achievement. With an indiscrimiating 25% of grades in 2011 A/A* you can truly grasp the repercussion of there being multiple exam boards, unlike the single-bodied IB. Therefore, with A-Levels, you are ensconced in the crowd and convention, whereas the IB makes you stand out lucidly.
EDIT: In a nutshell. I had a luxury of choice between two schools 5 mins away from my home which both house sixth forms. One teaches the IB while the other teaches A levels. I chose, in full confidence, the IB diploma which I begin in little over a month.