I've found that recruitment agencies that specialise in specific areas of work are more helpful than the big general ones. For example, I can recommend Charity People who just work with the charity sector. Eden Brown also found me a job.
For the creative industries, I also had success with Bespoke Careers who place people with design and architectural firms - including admin support roles.
(sorry, I'm not allowed to post their websites here, but you'll find them all on google)
Be warned though - they are unlikely to do anything for you until you've met with them face to face, and most of them require you to go in and take tests (for admin roles there's timed typing, word excel etc, and for more creative roles it might be indesign or photoshop etc). Also, I've found that the jobs listed on their websites often only represent a fraction of what is actually available.
For the arts, the Guardian newspaper (particularly on Mondays) is by far your best bet.
One final tip - if you have an address in the
UK make sure you put it on your CV, even if you're still in Canada. Most employers will discard any CVs from people not already in England, or even those not in London (assuming you're looking for work in London of course). I learned this the hard way, and then saw it in action while part of the recruitment process at two companies I worked for.
Good luck!