Finding the best UI/UX design agency in London requires considering factors such as expertise, portfolio quality, client satisfaction, and project requirements. Here are some top options:

1. Foolproof: Foolproof is a leading UX design agency based in London, specializing in user-centered design for digital products and services. They offer services including user research, UX/UI design, and usability testing, working with clients across various industries.
2. UsTwo: UsTwo is a digital product studio with offices in London and New York. They offer UX/UI design, mobile app development, and digital strategy services, creating innovative digital experiences for clients ranging from startups to Fortune 500 companies.
3. Idean: Idean is a global design agency with a studio in London, focused on creating meaningful digital experiences for brands. They offer services including design thinking workshops, UX/UI design, and digital strategy consulting, helping clients innovate and grow their businesses.
4. Kin + Carta: Kin + Carta is a digital transformation agency with offices in London, specializing in UX/UI design, product strategy, and technology solutions. They work with clients to create user-centric digital products and services that drive business growth and customer satisfaction.
5. Futureheads: Futureheads is a recruitment agency specializing in digital, design, and technology roles. They work with a network of talented UX/UI designers and can help businesses find the right fit for their project needs, whether it’s for freelance or full-time positions.
6. Red Badger: Red Badger is a digital consultancy based in London, offering UX/UI design, product development, and digital transformation services. They combine design thinking with agile methodologies to deliver user-centric products that meet business objectives.

Before choosing a UI/UX design agency, thoroughly review their portfolio, client testimonials, and case studies to ensure they have the expertise and experience relevant to your project. Additionally, consider factors such as communication style, project management approach, and cultural fit to find the best fit for your needs.