I can't answer your question about where all NPs work but I would have thought it was highly unlikely they work in the ICU. I've come across them in acute care but the patients weren't big sick. Also, in Canada, they are an extended class of RN and are registered with the provincial college of nurses with an extended scope of practice. This is unlike the UK which requires no registration to be a NP (I know they are looking into it). I'm not sure if you've done a masters to be a NP, however in Canada, a masters degree is mandatory.
I can assure you that Canada is tough to get licensed as a RN, even for nurses from the UK. It all boils down to education. Nurses in Canada are generalist trained, which means they have theory and clinical practice in pediatrics, maternity and mental health. Whereas, the UK is specialist trained for either adult, peds or mental health. Unless your initial education included all three of those areas, any of the provinces will make you do bridging programs to make up the gaps, costing $1000s before they let you register. Some provinces might be better and require less make up of educational gaps than others but I wouldn't know which ones.
It's of course not impossible to register here as a UK trained nurse but expect it to be a long and expensive process! I would have thought your NP experience would also not be recognized unless you have a masters. Even then, the college of nurses will likely find issues with it lol. The only way you can know for sure is to have your credentials evaluated. I'm sure others will give you more feedback. Sorry that my reply isn't more positive. Just thought you need to know what you're up against!