Did I do anything wong when making fries?
I haven't noticed regular potatoes being sweet when overcooked, undercooked, or even raw. So I don't think the sweetness was due to cooking time. 

However, according to  

http://www.finecooking.com/article/the-science-of-cooking-potatoes-2 

(and many other places on the internet), potatoes stored in the refrigerator will turn sweet as their starches convert to sugars at the cold temperatures.

Is it possible you kept these potatoes in your fridge for a significant time after purchasing them, or maybe they were kept refrigerated at Tesco for a while before you got them? If so that could explain it. I routinely keep my potatoes in the fridge, and when they go sweet, it's really noticeable. I happen to like them OK that way, but they are really quite a bit sweeter than you would expect.