All the energy that you generate and use in your home is free, obviously in the early days this goes towards the cost of the system but in the longer term when the system is paid for your getting free electricity year on year.
Reducing your Carbon foot print
Much of the electricity in the UK is generated by burning fossil fuels, this releases as much as 700g per Kwh. The electricity generated by the panels releases no carbon into the atmosphere, so its a great way to reduce your carbon footprint with clean renewable energy.
Currently the UK relies on several other countries to supply its energy needs with a large percentage of our electricity coming from nuclear plants in France and our Gas coming from Russia, we are subject to international politics. If everyone had solar panels on their house we could reduce the countries reliance of imported energy. This would also give the UK greater control of the prices of the energy we all consume.