I'm going to make heart cupcakes for St Valentines day too.
Recipe for swiss meringue buttercream is:
4 egg whites
250g/9oz caster sugar
250g/9oz unsalted butter, softened
a few drops of vanilla extract
Place the egg whites and sugar into a bowl over a pan of simmering water. Stir while heating to prevent the egg whites cooking.
Once the sugar crystals have dissolved, remove the bowl from the heat and whisk the meringue to its full volume and the mixture is cool - about 5 minutes
Add the butter and vanilla and continue to whisk for about 10 minutes - the mixture will reduce in volume and look curdled, but don't panic! Keep whisking until the icing has a smooth, light and fluffy texture.
Store in a refrigerator for up to two days, and re-beat before using.
This buttercream is not stable above 15 degrees C/59 degrees F, so not suitable for hot days or climates - so okay for UK then?