Morning Dim - I have no commercial interests in UK or the planet for plywood. Long certified straightedges are expensive but are worth it... But making straightedges from extrusions is good enough for your endeavour. I rarely use a torque wrench. Been making things since I was a youngster so most things by feel. Bolting rails to ply I have done in several ways:
1) Using round rail with the supported extrusion is a good solution. The support extrusion is broad and takes normal bolts or timber screws well. But the preload on round rail is light so positional hysteresis can add up. Prefer medium or heavy preload square bearings
2) Square rail I use timber screws (I like Kreg screws square drives. I have banned cross head screws from my workshop, do not use them) or capscrews. Using capscrews I pilot the hole and use a taper tap to create the thread. Being tapered means when you screw the capscrew or hex bolt in, it snugs up quite tight . If the screw is to be permanent I use PVA glue in the thread as a locking adhesive. They have never backed out or come loose... They still come out with a hand tool if needed. Use as long a screw as possible and you won't have any issues. Do some tests for yourself. For general screwing I use square drive screws intended for outdoor decks. Epoxy coated they don't rust.... zinc plated screws can rust after a couple of years I've found.
3) Don't use expanding PU adhesive. Its a pain and can change the dimensions of your parts over time as it keeps expanding. I have used single pack construction adhesive and I like it. Easy to clean up squeeze out with some kero and a rag... Just as good as epoxy for the job. Being solvated means it sticks really well. In breaking tests the timber breaks not the glue. Even the cheapest glue vs premium brands
4) Yes its time to get a good general arrangement done for review
5) if your really worried about the ply foundation for the rails then use an aluminium strip thats wide under the rails. Bonded to the ply after levelling or floating up to you. A 3x50mm wide flat plate is a good width for a 20mm rail... Since a rail is near an edge usually, I have also used a light angle that goes around the edge for more support. Don't know if it helped but it can also look better...
6) If there is a cabinet maker nearby get then to cut the long cuts on their table saw. Will be straighter then you can cut with a fence and a hand saw... I usually give a cutting list to a local company of its for important parts. The edges are square and true vs my hand cut stuff. Thats one reason I'm building an 8x4 machine so I can cut stuff up properly...

Keep at it... Peter

Heres my latest stack of parts for my 8x4 machine. I'm about to start assembly of the machine base....