By Amber Murray Pret a Manger may be a London lunchtime staple but it has made a firm push for international growth, with over a third of its stores and a quarter of its sales now in international markets. In its half-year results, the sandwich and coffee chain said its sales grew by 10 per cent year on year, to £569m. The growth was driven by Pret’s “continued expansion into new markets”, the company said, with Pret now open for business in 18 markets. “While the business remains proudly British-based and British-built, and continues to expand in nations and regions across the UK, a significa…