Night storage heaters often use a separate circuit to the rest of your house, this is so that you can be correctly charged for Economy 7 tariff. Any PV system can only be connected to one circuit, this would be either your night storage heaters or to the rest of your house. This would mean that potentially you could only use generated electricity for your storage heaters or for your house.
Another point is that PV works during the day, so your storage heaters would heat up during the day, rather than at night when required. As well as this, on cloudy days, days when you really want the PV system to work, they would not be heating up your storage heaters nearly as much as you would require. Also during the summer when PV systems operate their best, you dont need your storage heaters, and you’ll be wasting generated electricity.
The conclusion is, that even if possible, it would not be practical to have PV systems plugged in to storage heaters.