If you do business in a country, you have to accept that you're going to be subject to the laws of that country, and that clearly includes laws regarding competition. Facebook can probably appeal the ruling, but at the end of the day, if they want to keep doing business in the UK, they're going to have to comply with the decisions of the regulatory bodies there.

The US has blocked mergers as well, and has retroactively forced acquisitions to be terminated. An example was Grindr selling a 60% stake of the company to the Chinese company Kunlun in 2016, Kunlun purchasing the rest of the company in January 2018, and CFIUS issuing a ruling in March 2019 that a Chinese company owning Grindr posed a national security risk and forcing them to sell the company (although finding a buyer and completing the sale took another year).