You need to read the UKGBC consultation reports on definitions of Zero Carbon.
http://www.ukgbc.org/site/resources/showResourceDetails?id=180
The code is currently entirely voluntary. However, as often happens voluntary measures become regulated after an initial testing/bedding in period.
Except in Wales where all houses planned after 1st April 2009 have to meet Level 3. So its a regulatory requirement enforceable by LA Planning Depts.
Also Wales has an aspiration target of zerocarbon newbuild homes by 2011! Though is this case aspirational target can probably be read as ministerial slip.
For your essay it might be useful to note how the Code reinforces/extends other regulatory regimes such as Site Waste management Plans, bits of the Building Regs and flood risk planning.
Jonny