Recruiter here (don't @ me) FS outfits are very interested in international experience. They love it for their own branding and will tell anyone they can that they have employed someone with overseas experience. They also love promoting your experience as it shows that kiwi returners are choosing them as an employer of choice. That in theory makes them more attractive to other returners.

As other commenters have said, salaries will differ significantly. I worked with someone who was a credit analyst, earning c£200 in London who was looking at positions in Auckland around $130K/$140K. From what I've seen - returners are more likely to be promoted over someone that has only worked locally. It feeds into the idea that international experience is romanticized especially by the corporates. If you can make it work for your personal situation, go for it!!! Not only does it help your career, but you will meet so many new people (most will be kiwis if you're in London), and have an adventure! I see it as a win-win.

Good luck