I looked for the Knit Picks Wool but don't know if it's available in the UK.
Knit Picks is only available via mail order from the Knit Picks catlog or online store: http://www.knitpicks.com
Though any light worsted should do.
I have 150 sts MC, 150 sts CC
So, wait, have you cast on 300 sts total? Because if you are getting 5 sts per inch, that means you have a hat about 5 feet around (300 sts divided by 5 sts = 60 inches.)
If you have 150 sts total, you have a hat 30 inches around (150 sts divided by 5 sts = 30 inches), which is still much larger than the circumference of the average human head (about 21 inches).
You see, the pattern calls for 6 sts per inch, and not getting gauge means all the measurements are going to be thrown off. You either need thinner yarn or smaller needles, or both if you can't knit your current yarn on smaller needles.
Sorry for doing this all in inches but I don't have a calculator handy for converting to cm!
Anyway, to sum up:
1. Make sure you've not cast on twice as many sts as you need. (you only have to cast on in the MC)
2. Gauge is important. You need to devote some time to figuring it out or you will always have a problem when knitting from a pattern and you sub yarn or needles or simply knit more loosely or tightly than whoever wrote the pattern.