If you are looking for aluminium extrusion in this sort of profile there are a few companies in the UK that sell it, it is used in a lot of different industries.
Can't speak for any of them but RS Components, KJN & Valueframe are 3 that I know of. Just be aware that it comes in different wall thicknesses as well as different profile shapes so check that out & don't just buy the cheapest you can find as the aluminium wall thickness will probably be to thin for your needs.
I discovered many sources for the extrusion in Germany after I bought from item GmbH. Some of the sources offered to cut smaller pieces, but most sell only three meter lengths. Those who did cut to order did not break the edges or offer any other options like drilling and tapping holes.
I have no problem cutting, drilling, tapping, and cleaning, but I can't compete with the speed and accuracy of a CNC machine and process developed for finishing extrusion. I would, and have, done my own machining for small projects, but would not consider it for a large project. I could have ordered the standard three meter sections, but I might still be building my worktable and cleaning up the mess. Time is the one resource available to me that I don't have much control over. The additional cost from the same vendor, about €250, to deliver ready-to-assemble components rather than bulk extrusion was not worth worrying about.
You make a great point about the extrusion wall thickness. Each vendor offers different versions of extrusion, including hollow extrusion. I had considered using the hollow version for the vertical sections, but since they are load bearing, I stayed with the "light" version shown below.
The 40x40 extrusion was used for the all pieces except the top band.
The 80x40 extrusion is used for the top band for tool attachment.
