To a get a smooth finish on the buttercream, you'll want to make it quite thick in consistency but still spreadable. After spreading it evenly over the cake, use a hot palette knife (dipped in hot water then cleaned on a dry cloth) to smooth over the cake as best you can, scraping the excess icing off the palette knife as you go. Using a turntable really helps. Then after leaving the icing to form a crust for 30mins, gently smooth out any imperfections using a piece of parchment paper and your fingertips, or a fondant smoothing tool. To get the speckled effect, mix cocoa powder with a little hot water, then flick it at the cake with a pastry brush. Finally, decorate with mini eggs (or mini-mini eggs for an extra cute finish!).
Most US ingredients easily translate into UK ingredients, they just have different names, however some ingredients are special to the US so for those I have suggested the best equivalents, or ways to make your own substitutes. There are a few things you can pick up in places like Selfridges or Tesco's American Grocery Isle, but why go out of your way to buy them, when you probably have something just as good in your cupboard already.
See also: Cups to Grams Converter for Baking