Flank Steak is everywhere, it may go under the name Skirt Steak (except it is flank) Costco sell it and so does Morrisons, and many butchers. I have not seen it as anyplace like Sainsburys, Asda or Waitrose.
I have never heard of tri tip, so sorry can't help you!
We love flank steak in my house, marinated for ages and cooked slowly on the BBQ. My favorite...I might have to get some for the weekend! I miss true American style skirt steak though!