For the purposes of this post:
Canalphones = cheaper high street "in ear" phones" where the tips sit only at the entrance of your ear canal
IEMs = Inner Ear Monitors
IEMs come with a fit kit... interchangeable tips of various sizes and 'type' so for most people they can find the perfect fit. These tips sit deeper than the tips you get from canal phones. In the majority of cases you will get a proper seal and a better fit. Its not a certainty but I will bet that you will find a tip that fits better than the '3 standardized sized tips' you get with canal phones on your British high street
You could also consider the Shure range and also the Westone UM1 and UM2
I could go on... but see my rather lengthy post
here