Investing as an overseas non-citizen (US)?

My husband and kids are Canadian citizens and we're planning on moving to Canada within the next two years. We're all also US citizens, currently living in the UK (the kids and I are also UK citizens). I've found in the UK we have struggled with investing because financial institutions really don't like the US citizenship, and I'm hoping that will be less of a problem in Canada.

Is there any way I can start investing from the UK as a US citizen, and if so, what brokerages should I look into that can handle a situation this complicated? I'm getting nervous about retirement and want to make sure I'm contributing for the country I plan to actually need the money in.