They could possibly terminate them at Piccadilly as there’s already lots of services normally to the Airport, and shove the unit into Longsight Excursion Platforms or into Mayfield Loop whilst they wait for their booked slot north.
Currently, there’s arrivals at Piccadilly from Windermere at:
10:57, 12:47, 13:47, 16:48, 18:54 and 20:58
Departures back to Windermere from Piccadilly at:
08:48, 10:47, 12:47, 14:47, 16:54
It would be a long wait at some times of the day, but it would mean that the battery 331s wouldn’t block the platform at the Airport. Alternatively, they could run through to Wilmslow via Styal, utilising the turnback sidings there.