I don't know how edible onions are if they are succumbing to white rot. If the ones still look healthy I'd be tempted to cut my losses and get them up, chop them and freeze them (in an open layer, then you put them into a bag after freezing so they don't clump together).
But I've not had the problem so am not entirely sure if what I've suggested is a good idea.
The really bad news is that white rot persists in the soil for decades so I'd resign yourself to not being able to grow them on the plot for the foreseeable future. There is no legal treatment for it.
If you decide to grow them anywhere else e.g. at home, you will need to take very good care not to transfer the pathogen from plot to home. This could happen on boots, trowels, gloves, etc - you get the picture.