The disclosure indicates an ongoing feed rather than a single breach, giving a rolling window into high-level UK diplomatic, trade, and strategic communications. That yields both tactical annd strategic intelligence â" knowing the UKâ(TM)s negotiating positions, alliance tactics, red lines, and risk assessments before or during talks across the international stage.
Diplomatic advantage/control is gold dust for a state like China. It enables knowing policy shifts before announcement;ÂÂundermining UK positions in multilateral forums (WTO, UN, G20, COP, AUKUS-adjacent dialogues); Âidentifying which alliances are hesitant or divided, so as to exploit the seams.
There are obvious counter-intel opportunities - but those are likely to have been secured in a different channel.
TL;DR: China has had an ongoing and full view of UKs hand in the global poker game.