This is the best app for bus users in the UK. There are alternatives in the marketplace but this is the reference on both iOS and Android and I must say the WP app is just amazing. Links below for full UK or just London.
UK Bus Checker | Windows Phone Apps+Games Store (United Kingdom)
London Bus Checker | Windows Phone Apps+Games Store (United Kingdom)
UK Bus Checker | Windows Phone Apps+Games Store (United Kingdom)
London Bus Checker | Windows Phone Apps+Games Store (United Kingdom)