In UK sizes, a 2mm crochet hook is a 14, it doesn't not have a US size equivalent. Here is a chart with crochet hook conversions from metric to US to UK.
Us #10/j, uk
#4
A 0.9 mm steel hook is about a US size 13.
A 3mm crochet hook is halfway between a C and D size in US sizes.
In the US, knitting needle sizes are in numbers; crochet hooks use both numbers and letters.
A size 5 steel hook. http://www.crochetmagazine.com/hook_sizes.php
A U.S. size 7 yarn hook is 4.5 mm. http://www.crochetmagazine.com/hook_sizes.php
00 is a steel hook size. Manufacturers vary in their mm sizing. I found 00 hooks listed as 2.75 to 3.5mm If you have a particular hook and you need to know the mm sizing of it, I suggest you purchase a Crochet Hook Size Gauge Tool. The knitting needle ones do not give accurate sizing for a crochet hook. Annie's Attic sells these and I'm sure other stores do as well.
You can use another size that gives you the gauge you want.
It appears that in the UK hook sizing uses a numbering system (which is different than the physical metric size of the hook), which runs from 14 to 000--smallest to largest. In the US, "aluminum" hooks are lettered, beginning with "A" as the smallest hook and going on through the alphabet, generally with a "P" being the largest (although, I do believe that I have seen a pattern requesting a size "S"!). I have attached URLs which have charts to use for converting into US hook sizes.
Crochet hooks come in various sizes. To figure out a project that you might want to crochet, find the project, see what size yarn is needed, and then check with the pattern, and see what the pattern requires for a hook size.As a general rule "steel" hooks are used for thread-weight "yarn" and those hooks run from 0.6 mm to around 3.0mm. You know that you have a "steel" hook, because it has a number on it, in addition to a size given in mm. 'Steel' hooks run from size 14 (0.6mm) to 00 (double zero, and 3.0mm)Then the "aluminum" hooks run from around 2.0mm all the way up to 16.0mm or larger. An aluminum hook can be easily identified because there is a letter on the hook, along with the mm sizing. So, 'aluminum' hooks run from B (2.25mm) to S (19.0mm). I guess there could be an "A" size, but it would be around the same as a size Zero or Double Zero in 'steel' hooks.
It doesn't matter what kind of a stitch you are working into, you only work into the two strands at the top of the stitch. Insert your hook into the top of the dc, yoh, pull through the stitch top (2 loops on hook), yoh and pull through both loops on hook. These are US instructions.
yarn over, insert hook into specified stitch or space, yarn over, pull through stitch (3 loops on hook), yarn over, pull through all 3 loops on hook
The following is the American meaning of double crochet. The British system would call this stitch a treble. A British d.c. is an American single crochet. Double crochet is one of the three basic stitches of crochet: chain, single crochet and double crochet. To make a double crochet: 1. Put the working yarn over the hook (this is called a "yarn over"). 2. Insert the hook in the next stitch in the row below. 3. Yarn over again and pull that yarn over back out through the stitch. There are now three loops on the hook. 4. Yarn over again, and pull that yarn over through the first two looks on the hook. Two loops remain on the hook. 5. Yarn over again, and pull that yarn over through the remaining two loops. When you finish a row and turn, start a row of double crochet by first chaining three stitches. This chain-3 counts as the first stitch of the row. For a single crochet, chain only one to start a new row.