BANBURY, a little English town best known for a walk-on part in a nursery rhyme and as the eponymous origin of a fruitcake, is an unlikely fulcrum for the balance of power in the world of telecoms. But the “Cyber Security Evaluation Centre” set up there by Huawei, a Chinese telecoms giant, in 2010 marks a new way of persuading purchasers, and the British government, that equipment from the manufacturer that runs it can be trusted. It operates in close co-operation with GCHQ, Britain’s signals-intelligence agency, located conveniently just over the Cotswolds in Cheltenham. Its security-cleared staff,....