SIX BRITISH adventurers were "on top of the world" yesterday after they became the first team to row to the magnetic North Pole.  They were exhausted by their 28-day journey but were jubilant to have set a record they had been told was impossible. They rowed 450 miles from Resolution Bay in Canada, but found the last three miles so badly congested by ice floes and lumps of broken ice that they had to drag the rowing boat for several hours to complete the last stretch of the journey.  Jock Wishart, who has previously set records rowing across the atlantic, said the scale of their achievement had "yet to sink in". But he confessed to leaping about on the ice in excitement when it was confirmed that they had r