What we call worsted in the US is apparently slightly thinner than UK Aran/10 ply. If your yarn is DK, which is even thinner, using a larger needle would get you closer to the right gauge but may give a drapier, more open fabric. Again, it depends on your preferences.
If it's a UFO, do you have enough already knit to measure how many stitches per inch in the fabric? Whichever size needle gives you 5 stitches per inch should result in the right size, per the pattern. As one who regularly chooses a different yarn than the pattern specifies, I understand the pain of getting things to turn out the right size.