To choose a reputable local plumber in London, start by checking their qualifications and certifications with recognized trade associations like CIPHE or APHC. Read online reviews and ask for recommendations from friends or neighbors. A trustworthy plumber should provide clear, written quotes and be willing to explain their process. Don't hesitate to ask about their experience with your specific plumbing issue. Additionally, ensure they offer guarantees on their work and have proper insurance coverage for added peace of mind.