Britain’s long-term government borrowing costs were under pressure once more as another wave of worry over political instability rocked bond markets and kept the pound on the back foot. The yield on 30-year UK government bonds, also known as gilts, briefly hit a fresh 28-year intraday trading high at 5.815% before paring back gains, to stand 14 basis points up at 5.799% as Prime Minister Sir Keir Starmer came under mounting threat of a leadership challenge with Greater Manchester Mayor Andy Burnham lining himself up as a possible contender. With potential rivals positioning themselves ready to trigger a formal challenge for the Labour Party leadership, 10-year gilts were also suffering another sell-off, with yields firmly above 5%, up 14 basis points at 5.133%. Gilt yields move counter to the value of the bonds, which means their prices fall when yields rise. Rising yields on these bonds mean it costs more for governments to borrow from financial markets.…