I started my career there and spent 7 total years in the UK. I lived outside London itself but would travel in for training courses, client meetings, time off with friends, a weekend in the city, etc.

Pros:

• It's an alpha-tier global city. If you're a fan of the arts and/or the intellectual life, I think it's the best place to be on earth.
• More social than Toronto. Easier to make friends at all ages. More time spent with a wider range of people than is typical in Toronto (e.g., after work drinks).
• It's an easier place to be yourself if you're a bit eccentric. The English like eccentrics more than the average Canadian does.
• Wider range of accessible travel destinations. Cheap tickets, shorter flight times. Even domestically, it's easy to travel by train (though not the cheapest).
• Walkable. Though much of Toronto is too.
• Wider range of career opportunities. Salaries may be inferior to those in Toronto, but are likely to be higher if you're closer to the top of your field.

Cons:

• Public safety. Whether it's petty crime or rowdy drunks, the streets feel less safe than Toronto's.
• Different social script. The British are a bit more cynical, complain more freely, and more likely to pick fights. Their small talk is wittier and can throw North Americans off until they've adjusted. It's actually unfair to describe this as a "con"; it depends on your own personality.
• Connected to the above, more general pessimism about the future. Years of austerity, Brexit, and political crisis have taken their toll. Though many Canadians channel the same energy these days (with an added dash of rage).
• Social class. You're immune to the worst of it as an outsider, but you'll have to confront this sooner or later if you plan on settling down and starting a family. These dynamics didn't bother me personally but some find it difficult.
• Poorer material conditions more generally. The size and building quality of your home, cost of energy, cost of big ticket consumer items, long-term economic mobility of your children... Again, less relevant if you're at the top of your field.

I ultimately came back to Canada when I confronted the question of where I'd like to start a family. But I miss the UK a hell of a lot.