The guy in the video sounds Australian, but for some odd reason he says 'math' instead of 'maths'. So you're probably right... boo. Looking forward to the day when people don't stay in one place and startups begin catering to us itinerants.
Unfortunately, banking is a heavily regulated industry and the laws vary (in real and significant ways) from one country to the next. Big, multinational banks do it by opening different banks in each country, each operating according to the idiosyncratic laws of that country but all owned by a parent company that moves capital around as needed. Building a single thing for handling money that works across national boundaries without breaking laws is incredibly difficult.
Not some odd reason - he's an ex-pat in the finance industry. It's like how I had modified my pronunciation of 'cache' (although I refuse to change how I say 'data').
BankSimple are based in Portland, Oregon (so yes, US).
It's also interesting to note that in the U.S., companies are thought of as a standalone entity, and referred to in the singular, rather as a group of people.
So, it'd be "BankSimple is based in...", instead of "BankSimple are based in...".
When I first started reading things from the UK, I initially thought the writers simply used poor grammar :)