GIRL GUIDES met at the summit of High Willhays Tor on Dartmoor this week to hold possibly the highest picnic in Britain for Macmillan Cancer Support. At 621m, the tor is the highest peak in southern England, so the climb helped the walkers and their friends raise healthy appetites and good sponsorship. Taking part was Emma Hogg, 18, from Tavistock. She said: 'We've been given a great opportunity and big challenge to raise the profile of what Macmillan does for families throughout the UK. But anyone can go outdoors and enjoy a healthy picnic, so we hope others will join in and help the charity over the next two weeks.'   The young women from Devon are all members of a girl guiding team planning to hold The World's Highest Coffee Morning above Everest Base Camp this September. The expedition, sponsored by Somerfield, will coincide with Macmillan's flagship annual fundraising event of the World's Biggest Coffee Morning here in the UK.