In North West England lots of overcast weather is prevalant.
I have been monitoring many local solar systems to see how they perform in different weather / brightness conditions.
I realised that some more expensive solar panels like Sanyo HITS didnt perform very well in low light conditions and were being outperformed by many good quality inexpensive chinese panels.
When the weather became hot and sunny, Sanyo HITS solar systems gained on the cheaper Chinese systems.
Sanyo panels have a low temperature coefficient, thus have lower relative losses as the panels gain in temperature.
But as we dont get much prolonged sunshine, those systems arent outperforming like I would have anticipated.
Many Sanyo solar systems i've looked at around Greater Manchester and the Peak District are underperforming, many cheaper good quality Chinese solar systems with German or Austrian inverters. Many factors are likely to be influencing this.
UK Weather varies significantly as we move around our island. I dont think there is one optimum panel for the whole of the UK but different panels, for different weather micro climates.
If I lived in Cornwall or on the south coast of the Isle of Wight, i'd consider solar panels with lower temperature coefficients like the Sanyo HITS.
In North West England I dont see any virtue in paying the premium for them, because they dont outperform many other cheaper solar panels that extract energy well in low light conditions.