Are you looking to apply from Canada or once you arrive in the U.K?
I found it difficult to get job interviews when I was in Canada but it was quite straight forward as soon as I got to the U.K. Some of that might have had to do with my seniority level (I was a digital strategist at the time) though. That being said, my girlfriend who was a media buyer got a job while she was still in Canada through a recruiter.
If you’ve got your visa situation sorted out that would also go a long way to making things easier.
In terms of recruiters, there are probably thousands in London so you’ve certainly got your pick. We’ve hired people through Sumo Creative before and Propel, but it probably depend on what role you are looking (people will be better In the various digital specialisations) for and at what seniority level.
Hope that help!