CTEK only recommend using 1 x 2.5m extension with their maintainers. That said, I have 2x 2.5m extensions to get from the garage to the car and have no problems with charging.
Changing the cluster is not a particularly difficult job to do, a few screws and a couple of connectors. I think though, the cluster needs to be programmed to the car.
May be best to talk to a Jaguar Independent Specialist first before going ahead.