Wow, you have got to love that!! Weldable system with all non weldable parts. I had to look, thanks for the link and the laughRing Brundles and ask them. Have a look at this page: https://www.fhbrundle.co.uk/groups/56MYO__Make_Your_Own_Bollards where it commences "Our new handy range of weldable steel components..." yet every component linked off that page says not suitable for welding. One or the other is a load of bollards.
It is not clear from the picture if the collar is solid on the inside, or if it has a hollow where the outside has a bump. If it is solid, you could drill and tap the post and use 3 No. pointed grub screws to pin it into place (but the collar would have to be reasonably close to the end of the tube so you could get the allen key on and turn it).
...would it just need a few beads holding it underneath to keep a cleaner finish on top, provided it's a tight fit on the post I suppose?