After much browsing I concluded that the cost of buying commercial metal railings would easily run into several £Ks. Nobody offered standard dimensions to match my own needs.
My wife then suggested galvanized weld-mesh. As used for concrete reinforcement and dog cage fencing. Easily available and not expensive even in large sheets. I just need to come up with a strong enough framework which will withstand anyone leaning on it. Which means strong wooden posts firmly bolted to each "inside corner" of the octagon.
Now I've just discovered a fully galvanized fencing system with 1.5m [5'] tall steel posts ideal for my requirements. It will need a handrail added, for comfort and appearance, rather than leaving the narrow mesh top exposed. The handrail can even be used as further reinforcement.
There followed an exploratory tour to a discount DIY warehouse chain which stocked the stuff. I came away delighted with my 8 posts and 8 panels in the trailer. A huge relief to finally make some progress on this safety problem. I really didn't fancy making a wooden structure that was strong enough in the long term.
The 2m x 1m mesh panels proved to be both heavy and very stiff thanks to all the horizontal wires being doubled and welded. While the 40x40mm square steel posts are perfectly adequate for the task.
The greater length of the panels [than required] means I can trim them all to exact size. While the posts are tall enough to give me the freedom to choose how high to make the handrail independent of the mesh panels.
A neat bolt and plate clamping system means very easy removal of panels and posts if needed. They clamp the edges of the trimmed panels by their vertical wires. So are absolutely ideal for effortlessly turning the octagon's weird "corners" regardless of their precise angles. You couldn't ask for a more user-friendly, panel joining system for an octagonal building!
The inwardly angled, octagon geometry will also help to stiffen the structure. If it still seems too flexible it will be no problem [at all] to add central posts to each ~1.7m panel. Though I don't expect expect it to be necessary.
I shall be delighted to finally have veranda protection because I always felt rather exposed when moving around on the wet and often frosty and slippery planks. Fixing down some fine chicken netting might have been useful but I have no plans to spend much time out there. Snow clearing would be sorely exacerbated.
Access to the observatory floor is internal. So the veranda will only be used for maintenance of the dome. The overhang was thought desirable for several reasons, including appearance. Otherwise the building would look rather tall and narrow. Like an inflated sausage balloon. Besides, access to the outside of the dome would otherwise have been extremely precarious. With 15'+ ladders leaning against a nominally spherical surface!
I even bought a pair of 24" Bahco bolt cutters. The first I've ever owned. These will be used to trim the mesh rather than using a noisy angle grinder. Wearing ear defenders is no fun for hours on end. Not to mention the sparks burning holes in my work clothes. The Bahcos are very sturdy and go through the 5mm galvanized wires like butter. So I'm glad I didn't take the other, cheaper, no-name option in the shop.
Some angle grinding may be necessary if the cut stubs won't allow fixing to the post brackets. Though bending slightly longer, horizontal wire stubs is another option I shall look at. Bolt cutters don't cut perfectly flush due to the symmetrical form of their cutting anvils. I even thought of trenching wooden posts to cover the galvanized posts for a sturdier look. This might go better with a wooden handrail.