It is worth mentioning, as Standard & Poor's revises the UK's outlook to negative, that in the G7 only three countries - the UK, Canada and Germany - still have a AAA rating from all three main agencies.
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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.