Britain’s biggest supermarket Tesco (TSCO.L) on Wednesday posted a 55 percent slump in first-half profit, showing the scale of the financial damage wrought by discount groups and raising questions over boss Dave Lewis’ turnaround plan. In line with recent market data, Tesco showed a steady improvement in trading in its home business and it reiterated its full-year outlook, however it said the market remained challenging and it stood ready to invest more if needed.