Bit hard to answer, EMR might be interested as I believe looking into hydrogen trains was part of their franchise.
WMR seems unlikely as they have just got new diesel trains.
XC possibly for their 170 operations but still seems unlikely (where would they refill with hydrogen? XC )
Northern is again a possibility but looks unlikely to me, it had enough issues so introducing a new fuel source would just add complications.
GWR seems unlikely as they have got large amounts of cascaded 165/166s but could be a possibility being by the sea in some places.
Chiltern seems very unlikely, electrification would make a lot more sense for the route.