Welcome aboard!
This sounds like a specialized application with the short Y and long X, can you describe the application further?
A lot of stuff can be purchased if you shop around a bit. To be perfectly honest I have mixed feeling about using aluminum in a machines main structure though it can be fine for brackets, mounts and so forth. Aluminum isn't that hard to work with and frankly after you get the machine built you can always remake any pieces that offend your sensibilities. However when it comes right down to it steel isn't that hard to work with either.I am thinking to use Al extrusions, 90x90 mm (we have an excellent local supplier), a 2HP router for spindle, youtube-tested Chines rails, bearings, and lead screws.I am a bit at an impasse with the plates, angle brackets, motor mounts, etc., as I would need to make them myself from scratch.
For parts you can't purchase on line your best bet is to become familiar with local metal working and fab shops. For example a decent sheet metal shop will likely have a large press for bending metal and a shear and or band saw for cutting stock. They would be able to knock out some of the parts you need fairly quickly. Some of the parts might be considered temporary as sheet metal work isn't the most precise of activities. If not a sheet metal shop maybe a local jobber type machine shop that can make some of the components for you.
It should be noted that the one place where extruded aluminum T-slots makes sense to me is for someone without a shop and the unwillingness to seek out job shops for his parts requirements. I don't know if there are kit suppliers in the UK but it is worth looking into if local support is thin or you don't have the time to deal with job shops.
Oh one more thing not all T-Slooted extrusions are created equal, some are so thin or light as to be nearly useless in a machine tools structure.
Good quality wood products are very viable for machine builds. IF nothing else you can get crude solutions until your machine is functional enough to make proper components.Most videos I've seen gave me the impression that one needs a CNC to make a CNC! Al is the first choice, of course, and I've done plenty of Al milling on a Bridgeport when I access to one (had it for years), but now the only mill I've got is a DRO Sherline (some may know...). That thing is fine for indexing holes on 6x6" pieces, I can sure keep my pieces at 6x6 limits, but drilling and milling 1/4 and 1/2 thick Al Plates -- forget it. I do have a 1 HP Grizzly-like drill press, so I could index on the sherline and finish them off on the press. But slots, cutouts etc for adjustments -- forget it. My Sherline handles plastics *acrylic), and wood just fine. I would like to avoid plywood: I am in the UK, it's humid and temperatures fluctuate all over the place.
As for tooling to make the parts you can make everything by hand if you really want too. You would need to learn manual layout techniques and develop skills with the use of hand tools. It might be a bit "old time" but all machinery was built this way at one time.
You can get all sorts of plastic sheets from McMaster. However you can also get aluminum and steel sheets. So why not go metal first?Is there a good machinable plastic that would not absorb humidity like wood, not crack like acrylic, provide the rigidity to drag the 2HP router in XYZ, and available in flat sheets (like the 0.500" mirror-finish Al pieces one can get at McMaster)?
Recycled, especially free recycle, is often the best approach. However if you have a McMaster - Carr catalog there are all sorts of plastics and composite material listed including phenolics, PVC sheets, fiber glass sheets and so forth. I'd seriously consider though building your machine with temporary solutions to make the final solutions or enlist a local machine shop.I.e. a material that would be closer to Al than wood in strength, closer to wood in costs and machinability E.g. I have seen tabletops in my chemistry labs, made of some badass material... heavy as hell (>>much heavier than particle board), gives off a deadly smell once you drill it (and drilling it is a *****), gray dull powder once drilled, no visible fibers. So I could recycle some of these table tops. Never seen one crack or break, and I tried hard...
By the way just because a part is made out of wood or plastic does not imply that you can get away with less precision in layout. Your parts still need to be accurate and square where required, to prevent binding and interference issues. This is why I'm not convinced that using easier to work materials will leave you that much farther ahead. You send up suffering though a lot of screwing around (literally turning sees to force parts in alignment) when a part could have been made up square and to dimension in a machine shop.