They have an ongoing investigation into Google and Apple's practices with regards to app stores (and mobile platforms in general) [1], and have been looking for evidence on the topic of competition in app stores, and dominance.
While the UK is just one country, the CMA has a wide mandate to act on competition matters and has announced their preliminary findings on the merger of Facebook and Giphy, which (in short) recommend a forced sale of Giphy to prevent potential future anticompetitive moves, such as through terms of service. They have a particular current interest in digital markets, and are forming a new unit specifically to deal with tech antitrust issues (the digital markets unit)
While the CMA public consultation on the Apple/Google mobile platforms work has closed, their investigation team can be contacted via the email inbox listed at [1]. When engaging with regulators like this (for anyone not familiar), it is helpful if you explain why a particular move has harmed you, and can give examples or any available evidence about it, and explain the impact that it has had on you, and the impact on the wider ecosystem as a result.
I'm sure other regulators are looking into this, but for those who are unhappy about this situation, I'd urge you to contact a regulator. If your own national regulator isn't looking into it, there's no reason not to contact the UK one (or indeed any other), and explain your concerns and help them with finding evidence - often for policy and enforcement teams, getting evidence together is the hard part of their job, and I can say from experience they do read their emails.
[1] https://www.gov.uk/cma-cases/mobile-ecosystems-market-study
[2] https://www.gov.uk/government/news/facebook-s-takeover-of-gi...