Pete,
At the risk you might know all this stuff (in which case apologies) there is some useful info on this site
If you are buying stock from a UK supplier (who BTW?) he should be able to tell you what alloy it is, but generally if not otherwise specified it'll be 5083 or 6082T6 (there are very few aluminium alloys that dont have good machining characteristics, like 3000series and its very unlikely to be one of those)
Conventional milling (as opposed to high speed machining) says:
revs = 1000 * Vc/(pi * dia) where Vc is cutting speed of material in metres/min
which for most aluminium alloys is 60 - 120m/min giving a spindle speed of 6000 - 12000rpm on a 3mm cutter, so you are running a tad on the high side however high speed machining allows cutting rates up to 400m/min = 42krpm.
Feed rate Vf (mm/min) = Vz * revs * # of teeth, where Vz is feed per tooth in mm.
For most aluminium alloys a good rule of thumb is d/150 for roughing or d/200 for finishing, so assuming roughing Vz = 3/150 = .02 therefore feed rate needs to be around .02 * 18000 * 2 = 720mm/min.
Material removal rate Q in cc/min = width of cut * depth of cut * Vf/1000. You don't say what type of cut but assuming its 1mm wide x 3mm deep (edge milling) Q = 1 * 3 * 720/1000 = 2.2cc/min
This removal rate requires a certain power level
Pc = K * Q , where K is cutting power in Watts/cc
which for most aluminium alloys is 17 so P = 17 * 2.2 = 37W at the cutter, or around double that as input power, say 80W (~1/10HP) so its unlikely lack of spindle power is the issue.
Up your feed rate by 50%, or reduce the spindle speed, and you should get a better result...
but like all things these numbers are theoretical, you need to experiment with your own machine to see what its really capable of... rigidity becomes the key...
If you're in Europe why not come and visit the UK CNC Community at http://www.mycncuk.com