This is the way to go. "A complaint" in the financial world is a very specific thing, it's not just someone having a grumble about things. Financial Conduct Authority regulations mean that once a formal complaint has been raised the company is obliged to resolve that complaint within a specified time frame, and if they don't you can escalate it to the FCA, who on investigating can issue substantial penalties if the company is found to be at fault. All financial companies have internal teams whose entire job is resolving complaints before that happens.