I make my own modeling chocolate and I love working with it. Lauren Kitchens has a great tutorial on making it. You can find it on the following Jessi Cakes post, and there’s even some talk from UK bakers in the comments. IT seems that Amazon.uk does sell light corn syrup (not corn oil)
http://jessicakesblog.blogspot.com/2012/04/how-to-make-modeling-chocolate.html
Good Luck!
Jenniffer White, Cup a Dee Cakes - http://cupadeecakes.blogspot.com