Nurses prepare a bed on a ward at St Thomas' Hospital in central London January 28, 2015. Dearly-loved and overburdened, Britain's national health system has become the top issue for voters ahead of May's election, after winter brought headlines of ambulances queueing outside hospitals and patients languishing on trolleys for hours.The National Health Service (NHS) delivers care for free to the whole population from cradle to grave and accounts for a third of government spending on public services. Photograph taken January 28, 2015. REUTERS/Stefan Wermuth (BRITAIN - Tags: POLITICS HEALTH ELECTIONS)