Costco have a plastic offering around £40 per unit, 68kg per shelf rating. Not tried them but their metal offering is solid and reasonable money so might be worth a look if convenient
Having used both, I rate them both but the plastic ones are better in damp conditions. The chipboardsshelves on the metal ones get pretty rank, they’re also responsible for the metal shelves being incredibly heavy when packed.
The plastic ones can be manhandled onto a trolly and in/out of your car without snapping your fart strings.
The metal ones are much, much stronger but unless you putting engine blocks etc on them the plastic ones are more than strong enough for home /garden/ garage use
Be watchful with sizes if you are comparing the plastic ones – I bought some that I thought were pretty much identical to the costco ones from screwfix for what I thought was a good price – until I realised what I’d actually bought was a scale model of some shelves.