GC need their 180s for a while yet don’t they?
GC have no plans (publicly anyway) to replace their 180s so they'll need them for, er, a permanent period.
HT 180s remain the only possible contender other than 222s internally cascaded once Corby can go electric. The reality is that EMR are going to have to get a derogation for some of their HSTs to continue as there just aren't any fleets of suitable rolling stock out there that could replace the HSTs in the timescale available.
Plus not the wording used:
from May 2020, modern diesel trains will replace some aging HSTs.
The timescale is "from May 2020" not December and it's "some" HSTs not all. To be honest I'm flipping towards 180s being the option utilised. HTs four will be available from early 2020 which ties in nicely with a "from May 2020" start date.
Meanwhile we know that the Corby service isn't mentioned as getting new stock until December 2020. I mean that starts to narrow the field of two down to one...
That being said the wires are still supposed to be available from December 2019 so perhaps there will be some EMUs used in the interim period between December 2019 and December 2020 when the new permanent fleet is due to start.