It will depend on what each policy says. When I last shopped for policies, the handful that I sampled did indeed imply this—that you should buy a policy offered in your country of residence. Or if that is not feasible, one marketed to residents of many countries, including outside the home country of the insurer.
For example, I looked at a Bupa policy that appears to be available to residents of most countries. Here are some selected policy conditions:
2.3: If Annual Travel has been chosen, cover shall be valid only for trips of one month’s duration, as a maximum, unless extra travel days have been purchased. One month is defined as a period from eg the 5th in one month up to and including the 4th in the following month. The cover always starts on the date when the insured leaves the country of permanent residence and continues for one month. If the insured takes out the policy during a trip abroad, the one month period is still calculated from the insured’s departure from the country of permanent residence.
2.4: If Single Trip has been chosen, the insurance can only be taken out for an insurance period of maximum 12 months including any policy extensions.
4.1: The insurance shall provide worldwide cover, cf however Art. 23.1-27
4.2: The insurance does not provide cover within the insured’s country of permanent residence. This also applies even if the illness/ injury has occurred abroad.
And there is a definition of Country of permanent residence:
The residence where the insured has his/her permanent home or principal establishment and to where, whenever the insured is absent, the insured intends to return.
So Bupa will insure single trips of up to 12 months, but if you get the annual policy, they will only cover trips of up to 1 month.
A UK based insurer that only insures UK residents may either consider you a resident of China, or consider you on a trip that is too long to be within the scope of travel insurance (except perhaps with a custom policy).
Bupa would probably accept that your “Country of permanent residence” is China since you ordinarily return there after trips, and continue to have a job and home there while you travel, especially if you don’t also maintain a home in the UK. In that case they would cover you for trips outside China (up to 1 month or 12 months depending on the type of policy).