In the UK the standard reference is the "Aluminium Federation Handbook", which pretty much anybody dealing with the issue has on their desk. It covers all the current, and many of the older, alloy designations, with the main properties and conversions between alloys. I'm afraid that I'm not aware of an electronic resource as useful, and the Aluminium Federation's website at
http://www.alfed.org.uk/ seems to be down for maintenance at presence so I can't say if there's anything there.
G