The subsidy in its current form is broken - a large slice of the money people pay to get 4kW of PV panels goes to 'non-jobs' - essentially made up to help the building trade.
The cost of a rational rooftop system would be based on payback times of field-scale solar.
if you want to put a small panel on your roof - you can - but you only get the subsidy at the same rate it'd be paid to a more efficient scheme.
This would need the regulatory scheme to be largely scrapped.
If you want to install a 'solar shed' from B+Q, with a 1kW panel on the roof DIY, you can, and it will pay back after 20 years or so.
But getting subsidised so that spending 60% more than the cost of the raw panels upfront is a good investment was barking mad.