I think it's highly irritating that the Eureka timetable is being scaled down this way, so that it is only being delivered peicemeal. The supposed benefits of the restructured timetable are seeming ever fewer, although at least the London-York semi-fasts haven't been lost.
I would want to see the 5 180s going to a franchised operator rather than open access; the network is desperate for extra trains and I think the franchises should take precedence, as these are the operators most in need of extra capacity.
Personally, I'd like to see them going to either FGW, for the Portsmouth routes as has been mentioned above, which would free up 158 units to go elsewhere within FGW, or to TPE for either Manchester Airport-Glasgow or long distance north Transpennine services.
Of the two, I think FGW could make better use of them, as the problem with using 180s on TPE to displace 185s to work 170/3 diagrams is the aforementioned weight issues on the Hull services, which would mean that the 170/3s woud still be required so couldn't be cascaded anywhere, whilst 5 freed up 185s would be left with little to do (except bolster existing North Transpennine services). If it was possible to cascade the 170s (to Scotrail, releasing 158s to Northern or FGW) and use the freed up 185s more widely, it would be a more attrative proposition, but sending them to FGW sounds more sensible to me.
On the flip side, I can see the attractiveness of sending 4 to GC and 1 extra to Hull Trains (allowing them to undertake a wholesale refurbishment of theirs), to give both ECML open access operators standard fleets, the problem with that is the 3 GC HSTs that would be released are slightly non standard in their current condition in that they are only 2+5 formations, with no TGS either, so would be difficult to fit in with any operator unless they had a few extra mark 3s above the maintenance requirement lying around to insert in these sets to bring them up to standard, which is unlikely.
So send them to FGW!