I did a google search for Simplicity pattern for child's smocked dress and came up with this. I didn't look through the ebay sites but maybe one might be available in the UK?
https://www.google.ca/search?q=simpl...w=1280&bih=867
Here's the butterick pattern ones
https://www.google.ca/search?q=butte...w=1280&bih=867