UK long-term borrowing costs have surged to a fresh 28-year high and the pound weakened as Prime Minister Sir Keir Starmer’s leadership comes under increasing pressure. The yield on 30-year UK government bonds – also known as gilts – jumped as much as 13 basis points to 5.807% in Tuesday morning trading, reaching the highest level since 1998 as Sir Keir faced increasing calls from within his own party to quit. The yield on 10-year gilts also rose back above 5%, lifting by 11 basis points to 5.11%, but remains below recent highs reported last month. Gilt yields move counter to the value of the bonds, meaning their prices fall when yields rise. Rising yields on these bonds mean it costs more for governments to borrow from financial markets. The pound also weakened further amid the UK political instability while stocks on the London market dropped sharply on rising oil prices as the US remained in deadlock with Iran over a resolution to end the conflict.…