If main battery or its charging system fails, you may have IMA light on the dashboard ON, but car should run fine as any non-hybrid car.
Honda Jazz Hybrid uses hybrid battery to start the engine and for start/stop, but it also has standard starter fitted.
As mentioned above, in case of hybrid battery failure, only downside is about 70kg of unnecessary load and IMA light ON which may cause MOT advisory or failure.
On the other side, some forum members reported that the weight of the battery actually improves handling.