Hi Amanda, I’m in north east England, shortening over here is called Trex, you can find it near the blocks of butter in all the main supermarkets, however I have never needed to use shortening to make buttercream, crusting or otherwise, as far as I’m aware shortening in buttercream is used to get a whiter colour frosting or just personal preference, if I need buttercream I just make the generic uk buttercream, the one you can find on the side of Tate and lyle icing sugar, unsalted butter (or a mix of unsalted and slightly salted is quite nice and tones down the sweetness a little), icing sugar, vanilla extract (personally I use vanilla paste) and a little milk or water if it needs loosening up a little (I only use water, you never know how long someone is going to leave the cake sat out). Crusts perfectly every time.
In my experience with it it’s usually fine to leave out, although if it’s a particularly hot day, or you need it to crust/set up quicker you can put it in the fridge, I usually put it in the fridge for 10-15 mins before covering in fondant, just because I’ve found it makes it easier to cover when the buttercream is harder.
Hope that helps, and good luck
