It is miniscule in terms of jobs in the UK. Effectively non existent in the overall scheme of things other than instructing. PPL level flying completely blots out paid GA despite a user pays regime! A map of the country showing road, rail and airline routes shows why. Ignore the sticky out bits at the top and to the west of the UK. 55million people just in the main chunk. The entire centre spine of the country from top to bottom is a vast airway 50 miles across in places.
A few air taxi gigs, a smattering of aeromed, a niche industriette for the horse racing world, offshore patrol(fisheries and pollution). Everything else is turbine/jet, predominantly airline and executive and where the vast majority of the newly qualified go. That's the reason for the notorious UK (now JAA) exams - there is no GA layer acting as an effective filtering medium. Starter jobs are genuinely Boeings and Airbus for a very high proportion with modern glass turboprops and bizjets taking up the slack.
Jet Airline ops are out of all proportion to the population size but not just because the UK is a world business centre and one of the top five world economies. Our climate sees to that
A vast discretionary travel market has sucked in a very high number of Aussies, Kiwis, South Africans and pilots from all over Western Europe in recent years.
As to Canada - exactly the same situation as OZ. Similar population concentrated in tiny areas of a huge country. GA catering to the diaspora and a huge oversupply of pilots created to keep aviation schools and instructors ticking over. An endless cycle of undercapitalised, oversupply of aviation services ultimately dependent on subsidy in the great scheme of things.
Regards
Rob