The Prime Minister asked schoolchildren in Liberia what they wanted to be when they grow up. They said doctors and lawyers, to which David Cameron joked that children in the UK want to be popstars.
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The Labour leader did not say whether he would maintain Britain's nuclear deterrent programme.
Ukip leader Paul Nuttall also said he would want to get rid of Sharia courts.
Over 40,000 people picked up their packs to take part in the annual marathon in the capital.