This will take you to the Brimarc stockists in london, they may have some Flexcut products in stock but more likely will have to order them in for you.
I walked by there a number of times when I was in London in July, never when they were open though (I was in London for work, and my hotel, the Melia White House was close by), bummer...
But they did carry Flexcut from reading the window signs. Take the tube to Warren Street station, they're a five minute walk down Warren Street.