In a CAA letter dated 11 Jjuly 2014 it said:
Subject: Update - RTF to ATO and Changes to Regulations
There was very strong support for the deferment to allow RTFs to continue training for the PPL without ATO approval from 8 April 2015 to 8 April 2018 to permit time to do detailed work on alternative options for private pilot training outside of ATOs. Assuming the deferment is agreed, and alternative options are not subsequently agreed, RTFs would have to transition to ATOs by 8 April 2018 at the latest. The discussions on the alternative options have not yet started so we do not know what private training outside of ATOs may look like and what restrictions and conditions, if any, may be applied.
I have tried to find the letter on the CAA website without success.