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 going with a service like this on ebay. Basically you send in your complete battery unit to them and they will open it up and replace the battery with a brand new Li-Ion: