By Lars Mucklejohn A string of UK challenger banks have hit the headlines in recent months for grand plans to compete with their mainstream rivals on the London Stock Exchange. But executives looking to IPO have plenty to worry about. Who are the players?Firms that have said they plan to list include unicorn Starling, fintech lender Zopa and app-based Atom. Starling previously looked at listing in 2022 and 2023, but plans were put on ice after founder Anne Boden stepped down as chief executive last June. Monzo, which City A.M. understands is close to a funding round valuing it at around £4bn, …