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It is possible to submit a UK visa application abroad.
However, not all centres at all countries offer the super priority service for all type of visas. For example, the Bang Kok VAC does not accept superpriority applications for visitor visas (though priority appointments are available). There is also a risk that non-resident applications may be considered non-straightforward and the processing time is not guaranteed.
Please note that while the measures had been heavily relaxed in recent days (early December 2022), depending on several factors (place of origin, work/study history, or simply the mood or openness of the immigration officer), there may still be obstacles exiting China for non-essential purposes (especially to South East Asia).
I contacted UKVI who didn't say no, but read they're a third party.
UKVI stands for UK Visas and Immigration, which is the government agency within the Home Office charged with enforcing UK immigration rules, including making decisions on visa applications.
Visa Application Centres on the other hand are third-party agents (e.g. private companies TLS or VFS) on behalf of the UKVI to receive and support UKVI to process applications.