In an ideal world they should order something similar to the Hitachi 80X trains
No they shouldn't. In an ideal world (and, I would argue, even in this less-than-ideal one) they shouldn't be ordering any new trains right now because Network Rail should be funded to carry out extensive electrification works as per their decarbonisation strategy.
Yes, ideally Grand Central should have bi-mode stock for nowto make use of the wires we have.
But, if we add up all the class 800s, 802s and 805s we have 105 five-car and 48 nine-car 80x bi-mode sets. If you count the 33 class 810s as well that rises to 138 five-car bi-modes.
In addition, there are 42-72 diesel engines (I'm not sure if the 9-car sets have one or two) under the nominally 'electric' class 801 units. If some of the 801s were converted to class 803 spec (batteries instead of diesel engines) and others to class 800 spec by redistributing the vehicles with diesel engines under them between the units (eg. you take three 5-car class 801s (1 diesel engine car each) and reform them into one class 800 (3 diesel engine cars) and two class 803s (no diesel engines). If you keep the set lengths the same, rather than taking the opportunity to create some 7-car formations for example, the twelve 5-car 801s would become four bi-modes and 8 straight electrics, while the thirty 9-car 801s would give you either six or twelve 9-car class 800 bi-modes (five diesel engines under each).
We therefore potentially have a grand total of up to 202 AT-300 series bi-modes in the UK already. According to Wikipedia; 197 class 43 powercars were produced, so we actually have more 125mph bi-modes than we had IC125s. Similarly, tot up the Voyagers, Meridians and 180s and the total (119 units) comes to less than the AT-300 fleet.
Quite simply, there are already plenty of bi-modes in the UK, we just need to electrify more of the routes they currently work so that we can replace them with EMUs and cascade them to places like CrossCountry and Grand Central.