We already have the CBI complaining that post-GCSE school leavers lack the basic english and maths skills to become useful employees - reducing the minimum leaving age would just exasperate this problem even further. The education system is already struggling to imbue even the most basic of knowledge into the children most likely to leave school at 16 any reduction in time that these children spend in education would be disastrous.
One also has to consider the legal responsiblity issue too. At 16 years old an individual has at least some basic legal responsibilities that will allow them to be independant to a fashion. At 14, children are still the responsibilty of their parents and are considered differently under employment legislation. Reducing the school leaving age would mean a raft of new legislation which would, in effect, mean reducing the protections that young people have against unscrupulous employers.