I did explicitly specify a non grid tie unit. An off-grid unit that does not do any grid feedback is literally no different than plugging a UPS in to back up your computer in terms of the impact on the grid . It will accept the grid as an input to pass through but it will never feed power back into the grid thus a lot of the red tape goes away.
Maybe it's different in the UK but for the US a self consumption off grid inverter you do not need to even inform the power company much less be any type of electrician it's only if you are doing a grid tie inverter that will put power back into the grid that suddenly there's a lot of requirements.