Hi
You can look for a job while in Canada, but you can't actually apply for the work permit until you return home to the U.K. Also, as a non-resident, you couldn't just go through a local canadian paper and choose any job. Your prospective employer would need to apply for and receive a positive Labour Market Opinion (LMO) on the job you would fill - saying basically that skilled workers for that job are sufficiently scarce to allow a foreign worker to be hired.
I suggest you apply to immigrate under the skilled worker class. If it's not all you hoped for, you could always move back to the U.K.
If the OP is a student or under 35 she/he should look into a BUNAC work visa. It is a one year open work permit. cannot be extended.
see:
http://www.bunac.org/uk/workcanada/PMM