LONDON, July 7 (Reuters) - Former NATO chief and co-author of Britain's defence plan, George Robertson, criticised the government on Tuesday for not plotting a route to spending 3.5% on core defence and predicted a "frosty" response from NATO allies at a summit this week. Robertson, a Labour grandee who helped draft Britain's Strategic Defence Review last year, stepped up his criticism of Prime Minister Keir Starmer's defence spending plans, which included an extra £15 billion ($20 billion) to m