I hate to rain on everyone's parade here, but this mission isn't likely to happen soon. The paper referenced in the original post is a write-up of a case made to the call for ideas put out by the European Space Agency for future large missions, specifically looking for one to be launched in 2028 and another in 2034 (L2 and L3, in ESA-speak, with L1 being a mission to Jupiter and its icy moons, selected a year or so earlier).
Problem is, the Uranus/Neptune case didn't win either the L2 or L3 slot. A wide range of scientific ideas and mission concepts were proposed, aired publically, and assessed by a senior survey committee, before the two top-ranked ideas were approved by ESA's Science Programme Committee in late 2013.
And those two future missions will be a new high-energy astrophysics observatory for L2 in 2028 and a gravitational wave observatory for L3 in 2034.
The senior survey committee liked the science case for Uranus and Neptune, saying "The SSC considered the study of the icy giants to be a theme of very high science quality and perfectly fitting the criteria for an L-class mission", but then went on to say:
"However, in view of the competition with a range of other high quality science themes, and despite its undoubted quality, on balance and taking account of the wide array of themes, the SSC does not recommend this theme for L2 or L3. In view of its importance, however, the SSC recommends that every effort is made to pursue this theme through other means, such as cooperation on missions led by partner agencies."
So, it certainly won't be an ESA-led mission in the foreseeable future, but ESA could participate in a wider international mission if someone else leads it.
You can read the whole report here.