ITV News' Julie Etchingham accompanied the Prime Minister on a visit to Derby on Friday.
She spoke to him about varied topics including cuts to welfare and how best to help working families in a time of austerity.
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"My sincerest apologies to everyone who purchased tickets and were planning to come to the upcoming shows."
His comments at the Defence and Security Forum on Monday came after a £20 billion black hole in the budget for the next decade emerged.
The Foreign Secretary said he thinks the health service should be "at the very top of the list" for a windfall after the UK leaves the EU.