Worsted weight yarn equivalent?

I recently bought a crochet hat pattern, It required I use worsted weight yarn and a j size hook. I'm in the uk so I used double knit and size 6 hook, the hat stitches didn't look on the same size scale as the bought pattern picture and the pattern doesn't have a gauge guide, so I double stranded the double knit as it is very thin and now the hat seems too heavy. Can anyone help with this dilemma of getting a worsted weight equivalent please?