Britain’s long-term government borrowing costs jumped to their highest level for 28 years as City traders braced for a potential Labour leadership challenge from Andy Burnham . The pound is also on track for its worst week since 2024 amid growing concern over political instability. The yield on 30-year UK Government bonds, also known as gilts, lifted to fresh recent highs, rising by 16 basis points to 5.821 points. Ten-year gilts were also suffering another sell-off, with yields firmly above 5%, up 16 basis points to 5.155%. 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. The rise comes amid speculation that a new Labour leader could increase spending and government borrowing.…