Britons could soon install balcony solar panels in flats and rental homes

Yes, its kinda complicated and there are lots of reasons. The solar advocacy groups fought a lot to make it so that you dont even have to ask/notify your landlord or energy provider for these <800W installations

• Feeding in a lot of power (and especially quickly switching it on and off) can create grid instability (because the electricity grid is stupid and designed for large centralized powerplants)
• At some point (really big commercial installations) you are even required to install a remote disconnect system that the provider can use to turn off your installation if the grid is overloaded
• You dont get the same amount of money for selling energy then buying it, which means with high power installations the provider loses money if they dont install a smart meter (meter that keeps track of how much flows out and into the grid)
• With <800W installations the provider can still install a smart meter if they want, but you dont have to notify them of your installation

I hope i didnt mix something up here. Obviously read about your local laws if you install anything.

Unregistered as in you but the solar panel and inverter and just plug it into a regular wall outlet with a normal plug to back feed the house.

Before anyone suggests it's a suicide cable, the inverter have grid sending and won't back feed power without the grid electricity being sensed.