What is a meaning word "a"? [closed]
“A” is just being used normally as an article.
If we shave down the sentence to just the necessary words we need to answer this question, we get:
Johnson allows TikTok a London HQ.
All this means is that TikTok is now allowed to build an HQ in London.
You could also think of this as:
Johnson allows TikTok [to build] a London HQ.