By Ben Hirschler LONDON, May 29 (Reuters) - Cannabis capsules failed to slow the progression of multiple sclerosis in a large British study, dealing a blow to hopes that the drug could provide long-term benefits for patients with the debilitating nerve disease. Despite promising signs in earlier, shorter studies, researchers found patients who took capsules containing tetrahydrocannabinol (THC), a key active ingredient in cannabis, fared no better than those given a placebo.