Seems to me there are two different ideas going on here.
The first is buying co-op from the US so that we have a larger range of products; the other is a discount scheme for suppliers' current ranges.
The former helps open up the market, the latter just gives us a couple of pennies reduction.
There are a number of products I see on non-UK websites that I'd like to try at, but "try at" isn't enough for a UK supplier to stock the items. If, however, there were several of us who wanted to try certain items then someone (either a co-op team leader or a helpful supplier) could get the items in and distribute them as required.
One case I've mentioned before is plates. Outside the UK, there are different options. In the UK there is just one - and it doesn't look pretty.
I know some suppliers have tried to sell alternatives in the past - BMS used to sell plates, including plain white ones, but having a choice of just the bog-standard non-pretty one or heavy-as-concrete plain white ones, just doesn't cut the mustard.
I'd like to try out other types, but I'm not prepared to buy in from the US or elsewhere because I'm a small business and will probably be palmed off with anything they can't sell to their big customers. Small businesses like us have no voice. Only those who buy by the truckload can complain loudly enough when things go wrong.
I once contacted a company about buying coasters in bulk, but the reply was something quite snooty about "don't usually sell to end users" and "minimum quantity would be at least 250". Well, I was looking at quantities of up to 500 but their snooty attitude put me right off, so I didn't bother with them any more.
The idea of all coming together to make our voices louder is a good plan, but the administration of a co-op thing and "who's to blame when it goes wrong" would be a nightmare for whoever decides to do it.
And so we're left with the "suppliers discount scheme" - which is fine, so long as that supplier supplies the supplies you want to buy!

If supplier X offers 20% discount, but they don't sell what you want to buy, then the discount is meaningless.
How far does the discount scheme go? First supplier X offers a discount, then supplier Y offers a discount and, not to be outdone, supplier Z offers a discount... By the time you've finished everyone's offering the same discount - so there's no incentive for anyone to go anywhere in favour of anywhere else. Suppliers would soon realise this and then remove all discounts.
It's a right old kettle of fish.