As someone who's made the jump from Vancouver to London hopefully I can help a bit.
I think the biggest thing to figure out is what type of job you are actually looking for. Do you want to work in a marketing department or do you want to work for an agency? If you want to work for an agency, what part of the agency do you want to work in, Account Management, Creative (Art or Copy), Planning, New Biz, Social/Digital (depending on how the agency is structure) or Production Management? If you can figure those two things out then you'll know who to target in your job search which will be massively helpful.
In terms of actually getting a job, the upside is there are lots of entry level jobs around (although that might change slightly post Brexit), the downside is they don't pay particularly well. If you don't have much experience an internship is always a good route. If you do some research you can generally find some internships that pay which is definitely a bonus. Sometimes smaller agencies (or ever some larger ones) will also higher execs out of uni.
The trickiest bit is it will be hard to find a job here before you arrive, especially for a junior or entry level position. Most employers won't hire from North America without previous experience, and it's even less likely if you don't have your visa sorted. Most of the time that means you've got to save up some money and take the leap and then try and make it work once you get here.
For Visa, the upside is that if you are under 31 (at the time of application) you can apply for the Youth Mobility Visa. This Visa allows you to work in the UK for any employer for up to two years. After that two years you will need your company to sponsor you on a Tier 2 Visa in order to stay, but hopefully as you've done such an amazing job that's not a huge issue. You may also a UK passport already, or UK ancestry, so maybe that's not an issue.
In terms of actual salary, as you eluded to, London is one of the most expensive cities in the world to live in and rent is the biggest killer. Entry level marketing jobs tend to range from £20k-£25k per year which if your rent and bills come to £600-900 a month can make things very hard. As a result most people in London (even those on decent salaries) don't level alone. Early on in your career you are probably looking at a house share with quite a few people in it (which can be both a positive and negative!) If you can make do living in a closest with 4 or 5 other people then you might be able to bring your rent down to the £400 per month range. I do know lots of entry level staff who work second jobs so they have money to actually go out.
I'm definitely not trying to bring you down, I'd highly recommend you trying to make the move. London is a fantastic and I truly love living here. It is however very expensive to do so, and rent can be an absolute killer (more so than even Van). That being said, there are always ways to make it happen you just need to work hard and be able to budget.
On the plus side, if you are young, the Visa situation is actually pretty easy which is nice!