LONDON (Reuters) - Marks & Spencer suffered a drop in first-half sales and warned of an increasingly tough trading environment after disruption from the latest attempt to reinvent Britain's most famous retailer hit both its food and clothing businesses.After more than a decade of failed turnaround programmes, the 134-year-old retailer is now targeting sustainable, profitable growth in three to five years by shutting some of its less-successful stores and warned that sales were unlikely to..