The UK actually has good conditions for solar PV generation. The three factors which affect solar generation primarily are the angle of the panels, their orientation, and shading (if any). A 35 degree pitch roof, facing dead south will generate around 1000kWh annually per kW of panels installed (so a 5kW system would be expected to produce 5000kWh over the course of a year). In the non-optimum case, a east facing roof for example would produce around 30% less.
Shading is another factor in solar PV output – the panels act as one giant panel – so any shading on one panel will effect the whole array. This is something we can check on survey to ensure there will be no issues. In the case of something unavoidable like a chimney or a very large tree, we can use a special type of inverter which can work with partially shaded systems.