Since Terri is busy...I'll jump in and answer a few questions..
1. Yes...you can participate from UK...we'd LOVE to have you!
2. Your center can be rectangular...it can be any size up to 24" (finished) either or both ways. Just keep in mind that the 1st round will be 3"...so if you make it a dimension that is divisable by 3, it will make it easier to work with...other than that....get creative!
3. Generally when shipping international you can use the flat rate envelopes for the 1st couple of rounds, then you end up having to use the med. size flat rate box or the flimsy regular priority envelope due to the size. It usually runs around 20-25 bucks to ship near the end not including insurance.
I see the list is still growing :shock: :D