GENEVA (Reuters) - A United Nations (news - web sites) anti-torture body criticized Britain on Friday for the "unsatisfactory" conditions in its prisons, including a "substantial number of deaths in custody," and urged it to act at once to improve them. The U.N. Committee Against Torture, reviewing British compliance with an international treaty outlawing inhuman and degrading treatment, also expressed concern at parts of an anti-terrorism law which allows "potentially indefinite detention" of foreigners without trial.