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If you only want one size and fairly large, i.e. 15 mm plus . A hole saw is probably your best bet, a selection here at screwfix.
http://www.screwfix.com/p/starrett-20mm-holesaw/95906
These also work but I am not really a fan, depends what you need positional and size accuracy wise. https://www.amazon.co.uk/Q-Max-Sheet-Metal-Punch-QMA20M/dp/B0055KJGW8
Preferred for one off holes.
freezing77-
A hole saw is probably your best bet
↑This, ish. Drilling sheet that's less thick than half the drill diameter with a twist drill is pretty hard to get right even on a proper drill press. A hole saw will at least mean you're not involved in a wrestling match, although a drill press would still be a good idea.
Some screw hole punches (e.g. http://www.ebay.co.uk/itm/Screw-Hole-punch-Hole-punch-Hole-Puncher-Puncher-RUKO-12-7-63-5mm-ALL-SIZES-/401059296943) will handle 3mm aluminium and will make neater holes than a hole saw, particularly if the hole saw in question is on the end of a hand held drill. The pilot hole can be as wonky as you like, as long as the screw goes through the punch will align flat to the material.
This style of deburrer is easy to use
http://www.ebay.co.uk/itm/Neilsen-HSS-Taper-Deburring-Tool-Reamer-Pipes-10mm-34mm-Pipe-Steel-Copper-14B-/370978352112But you can deburr the holes perfectly well with a Noga with a bit of practice
http://www.ebay.co.uk/itm/NB1100-Noga-Deburring-Tool-With-2-Blades-UK-SELLER-/182366240699
gbj_tester
Thanks.
They look especially good as I could order it to exactly the right spec to cut down on... cutting.
Next question is what type of drill bit would be best to drill big round holes?