I used to work at a Kia dealership a few years ago. They are a Korean company, and when I worked for them all of the cars were imported from Korea, although I do believe that some are produced in Europe now.
Like anything else - you get what you pay for. They are OK cars - the build quality can be very plasticy, the engines are usually Ok and are reliable, although in my experience can be very underpowered on some models. The basic models can be very basic (they sometimes do a special ultra cheap model in the range with no radio, power steering, only available in flat red etc to advertise very low prices - ther are only very limited availability of these too)
They come with good warranties and are good value for money.
I think that you may have got things a bit wrong though - their cheapest car at the moment is the Kia Picanto - and the cheapest model is the Picanto 1.0 which according to their website is �6195 on the road, so with your trade in you are looking at �4195 (the price they advertised probable showed the scrappage price), which is not much to pay for a brand new car. More details are here
http://www.kia.co.uk/New-Cars/Range/Compact/Pi canto/Scrappage-Scheme.aspx
Good Luck