It can can be confusing if you can't get the correct tension for a crochet pattern. Here is a rough guide as to which UK yarns suit which crochet hook:
2ply - 0.75 - 3.00mm - a very lighweight yarn
3ply - 2.25 - 3.50mm - ideal for baby clothes
4ply - 2.50 - 3.50mm - Also ideal for baby clothes when looking for a lighter weight but don't want to go for a 3ply.
Double Knit (DK) - 3.50 - 4.50mm - very versatile, quick to crochet and very popular for baby cardigans, sweaters etc.
Aran - 5.50 - 6.50mm - Very quick to crochet and ideal for cardigans and sweaters either for children or adults.
Chunky - 9.00mm and above - Very thick yarn which is easy to crochet with and very quick to make up.
Experiment with hooks within the guidelines above until you get the correct tension.
If the recommended yarn on the crochet pattern is a named yarn as opposed to the weight is no longer available, you can get a rough guide by looking at which size hook is used on the pattern to get an idea of the yarn you can substitute it with.