Unless something's changed recently then I think the Class 97/7 referred to is a pair of former Class 501 cars radically converted to become a battery loco in the 1980s, rather than a reference to the LU bespoke battery locos.
AFAIK the LU battery locos aren't generally used on Network Rail lines, other perhaps than where they are immediately adjacent to LU lines. Recharging the LU battery locos would be a problem, as they need either the four-rail supply or (I think) the shed plugs found at LU depots. Would give Network Rail control kittens if they had to rescue a pair of dead LU battery locos from somewhere on the Great Northern
Pre-1976 when the GN&C was (an isolated) part of LU the route back to home territory was from Drayton Park transfer sidings, down to KX, through the York Road Curve, Widened Lines to Farringdon and then use the crossover from the west end of the Up WL platform to the Inner Rail Circle/Met and on to Neasden.
I'm struggling to think of a quick route now - both the York Road and Hotel Curves are gone, as well as the link between the Widened Lines and LU at Farringdon. I guess the quickest route would be via the NLL from KX Freight and Willesden to the Bakerloo - and from there to one of the depots that looks after battery locos.