To the OP I would say try not to worry too much about the sets poor handling of SD, aside from turning off unnecessary picture 'enhancement' features there is little else to be done. I have a cheapish LG 47" full HD set, which also isn't great with SD material though it's not too bad.
I don't think any HD or UHD TV can make SD material especially Freeview look all that great. I think rather than worrying you bought the wrong TV, the better option is to try and feed the TV a good source to begin with so it doesn't have to do so much scaling.
For example try to use blurays instead of DVD and depending on how important live TV is to you consider the merits of either satellite or cable (as these carry FAR more HD services than Freeview) or instead a Netflix/Amazon Instant Video or Now TV subscription. these are a lot cheaper and no long term commitment, while still streaming in HD but need reasonably fast internet connections.
Of course all these things have a associated cost but your TV (and eyes) will thank you for it and you are ultimately making much better use of all that available resolution on your TV as it seems a bit of a waste to buy a, even budget, 4k set just to watch SD material on...