The UK is a bit far away from Turkey
If I were to make a list about worst place to live, probably Turkey would be in top 10 just after Iraq and Afghanistan:) You are lucky, living in UK should be like living in a dream. I was working in USA, but after my company make an agreement with Turkish government, sentenced to live here for another year(I am Turk right, best candidate to work there). The only good thing is I am able to sell BTC with much better price (local exchange price is good) and of course free electricity
If your solar system is a standard grid-tie system, you need to factor the times that the sun is not providing you power. I assume that 5 KW is the 'nominal' power, you can get an estimate of what it would actually produce by looking at actual systems in your region at (http://www.pvoutput.org/). Unfortunately such systems deliver the full nominal output only for brief times.
Cheers