Will "the government" in power in those future years, be it of different political hues, have any different views on that particular matter?
I am no fan of any of the main political parties in the UK. But given there was 259 units produced outside the UK between 2002 - 2014 and many people in all political parties questioning as to why that could not have gone to any UK manufacturing sites, you can see why any future orders there was stipulation of at least final assembly to a certain percentage here in the UK. Since that period, there seems to be an almost insistence that even though companies can make some units abroad outside of the UK, there has to be a percentage of final assembly work done here in the UK written into the contracts. All you have to do is look at the details of the contracts for Hitachi to build the IET C;ass 80X AT300 and class 385 AT200 trains or CAF with it's Civity trains.
I know that some will ask well, why has that principle not happened with Stadler, when it comes to the class 68, 88, 231, 398, 745, 755, 756 fleets. Well, although there is 173 items ordered from Stadler, the orders themselves are only small orders and at the time the facilities for Hitachi where i the throws of AT200 & AT300 production with Alstom (Bombardier) doing Aventra production with the class 345 for Crossrail Elizabeth line. However, I may stand corrected on this, I do believe that there is a fair amount of components that came from the UK that was sent to the Stadler facilities to be making those orders, so the orders where done with some UK productivity.
Now this is just my point of view, but I believe that if the current Government stays in power (I.e. Conservatives) that it will probably remain at 50% - 75% of Production done here in the UK if capacity exists.
However, if the Labour party get into power I can see the final assembly and UK parts nearing the 80% - 95% mark of the productivity. This would in turn mean, more jobs at the factories and potentially more jobs for companies within the UK to be working with those factories on items for their train products.
The above is just my view and I would not like to guess where any of the other parties may lay on this issue, other than perhaps you would see the Liberal Democrats taking a position halfway between the two scenarios above.