The 68s are going to TPE along with new stock currently being built by CAF.
From this months Railway Magazine:
STOP PRESS
CAF has confirmed it is to build 65 new carriages for TransPennine Express, which will be hauled by DRS Class 68 locos between Liverpool and Newcastle from 2018.
More on this next month.
https://www.tpexpress.co.uk/news/2016/05/more-new-trains-for-the-north-and-scotland/
http://www.railtechnologymagazine.com/HS2/tpe-completes-new-fleet-procurement-with-25-caf-trains
With 13 5-car loco-hauled sets, each with a driving trailer, no doubt 13 locomotives will be required. These will be class 68 initially but with the prospect of electrification of the route and upgrading to 125mph the class 88s could find work here also.
What is the timeline for electrification of Newcastle - Liverpool? Found it, by 2022. So 68s for four years then 88s would be my guess.