(FILES) Britain's Prime Minister Keir Starmer (R) and US President Donald Trump (L) shake hands as Starmer finishes his opening statement and hands over to Trump at a business event at Chequers in Aylesbury, central England, on September 18, 2025, on the second day of the US president's second state visit. US President Donald Trump on April 15, 2026 has threatened to renege on a trade agreement with the UK, limiting the impact of US tariffs, as he again criticised Britain's lack of support in the Iran war. (Photo by ANDREW CABALLERO-REYNOLDS / AFP via Getty Images)