It seems unlikely that this deal would create a monopoly.
HKEX (which wholly owns the Stock Exchange of Hong Kong) is a publicly listed company. The HK Governeent owns 5.4%[1].
[1] https://www1.hkexnews.hk/listedco/listconews/sehk/2018/0319/...
https://www.predictit.org/markets/detail/5429/Will-the-UK-of...
It seems unlikely that this deal would create a monopoly.