Operationalising LLMs for Compliance-Critical Letter Writing in Financial Services

Large Language Models (LLMs) are transforming how financial institutions tackle labour-intensive tasks like drafting personalised, regulation-compliant letters. This paper presents a deployed LLM-based system that has generated tens of thousands of complaint-resolution response letters for a major UK bank since November 2024, following its demonstration of a 30% boost in letter quality and a 62% reduction in drafting time during the development phase. It has already doubled the bank’s three day complaint resolution rate, improved customer satisfaction, and reduced manual workloads. In this paper, we detail our end-to-end LLM framework for regulated complaint handling, our continuous “LLM-as-judge” approach for compliance monitoring and prompt optimisation, and our large-scale deployment strategy. In addition, we share lessons learned from overcoming scepticism through iterative development and user engagement. We also discuss the technical architecture and operational insights that have enabled a secure, robust, and future-proof deployment in this high-stakes domain.