What Should I Look for When Hiring an iOS App Development Agency in the UK?

I’m researching the best approach to hiring an iOS development partner in the UK and would appreciate some advice from people who have experience with mobile app projects. What should I look for when comparing an agency, including technical expertise, portfolio, pricing, communication, development timeline, and post-launch support? I’m particularly interested in recommendations for businesses evaluating an iOS app development agency in UK and want to understand what questions should be asked before choosing a development partner.