One issue is that the government is already in trouble for not meeting pollution targets..
Air quality in urban areas: Key issues for the 2015 Parliament - UK Parliament
... in February 2014 the EU Commission began infraction proceedings against the UK. These could take several years and could result in the UK Government being fined. Should this occur, it would be the first time the UK Government has been fined by the EU for breaching legislation.
If the government just pressed on regardless their decision could be challenged in court. It probably will be anyway but a lengthy legal challenge will make a 6 month delay seem short.