comparing living in Toronto and living in London is like comparing a granny smith apple to a red delicious apple. They are both markets however they are 2 completely different markets.
You might want to start looking at job recruitment websites to see what is the "average salary" for someone with your qualifications. I find industry-related websites are the most helpful (if you are in advertising/media, I can point you in the right direction). Just because you make $50,000 here doesn't mean that you will be making the equivilant in London. You might find that you might only be making £30,000 (approx. $44,000) tops.
As for costs of living, it will depend on what you want and where....the different boroughs of London will offer different price points (just like here). It's difficult to say....here are some websites:
http://www.workgateways.com/working-cost-of-living.htmlhttp://www.ukstudentlife.com/Prepare/Cost.htm (this one is for students but still gives you an idea of the cost of living)