You seem surprised that UK law holds in the UK.
Then you've misunderstood.
Meta is operating in the UK. Just because it is an American company, they have a presence in the UK. The UK government is demanding they sell Giphy, they don't _HAVE_ to follow that demand, but they'll most likely be banned from operating in the UK, which to them is more valuable than keeping Giphy.
This is no different than a UK based company operating in the US and having the US tell them to sell a product they acquired similarly.
As a matter of fact, the US does this stuff all the time.
You and I are talking. There's no "only one" as though there's a crowd of people present. Try and imagine you're not at the head of a baying mob.
I said that forcing a separate product to be sold in a territory by the rulers of that territory is very different to the rulers of a territory forcing the undoing of a merger in a totally different territory.
If you disagree, and think they're the same, then say why. If you don't disagree, then say that. Either way, please stop making points that I'm not even necessarily disagreeing with, but aren't relevant to what I'm saying?
Has the US actually tried to block a corporate merger between two non-American companies before?
There's a difference between a build tweak for another market, and a non-US country reaching into the US and blocking an acquisition of one US company by another.