PC Mick Wass, his rider during his time at the Met, said, "Chiswick has always liked to think he is the one in charge, but he has always looked after me."
Jeanette Allen, Chief Executive of The Horse Trust, said: "Chiswick is a real hero and it is a privilege to be able to provide him with a dignified retirement after spending a lifetime bravely serving the people of London."
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Donald Trump is to visit the UK on July 13 and hold bilateral talks with Theresa May, Britain's ambassador to the US has said.
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The most hotly-tipped date is July 13, following the Nato summit which the president is due to attend in Brussels on the previous days.