The Victoria Pendleton Somerby ebike has a Bafang rear hub motor built into the rear wheel. The 2020 model has a Bafang 36V 250W Motor.
So basically you'd want a 36V battery. The problem is it looks like the controller is located inside the battery box on the rear parcel rack. You'd have to open this battery box and look for the controller. You'd then need to figure out if your new battery will plug into the controller.
I think you're best bet would be looking for a battery on ebay or going with a service where you send in your old battery to them and they will open it up and replace the cells with a brand new Li-Ion: