Forgot to subscribe to this post so i didn't see your question until I was randomly surfing the forums.
The biggest issues is getting the cores back from the UK due to the shipping costs.
Honestly I think a group buy would work the best. I could probably get enough sets of cores that they wouldn't have to be returned and instead of the standard core charge I could just pass the cost of the cores along. Core charges are typically higher then the cost to replace the parts simply because it acts as an incentive to actually return the cores, which in the end saves time and ensures proper supply. I could either ship all of the set to one central person who then ships them out from there or ship them to the individuals.
There are a few approaches that can be taken but it really comes down to the amount of interest there is.
