Reasonably sure that the majority of those who have taken this employment opportunity to help train the Chinese armed forces are conducting a prohibited act. The potential criminal disposal of such individuals is a little tenuous though - there is a legal thread that could be pulled by the CPS but it relies upon some rather weak legislation just to prove that this prohibited event is, in itself, criminal. Still, it is a thread that could be pulled but I highly doubt it and hopefully those who have taken their skills to China will have paid for suitable legal advice.
I am presuming all those exporting their military skills did so at least 2 years post-exit, as that rule applies to all officers on leaving (and is not unique to the Armed Forces). To date I am only aware of civil action against those who transgress the 2-year rule but it can be a rather expensive experience.
For the majority (perhaps all of them from what I understand) having left as 'retired officers' do continue to hold a commission. The line from the JSP to QRs (KRs) is clear and precise:
QR J2914 refers. Officers who leave regular service must not, so long as they continue to hold a commission, or to draw retired pay, enter the service of any government outside the UK, or organisation sponsored by such a government, or any international organisation, without the consent of the MOD.
Either when directed by HM (aka 'Removal') or a 'granted' individual request for 'Resignation' post-service this equals the loss of all pension / gratuities / lump sum. In no way is this element a criminal conviction and it would not need to be disclosed; it is rather expensive though, given the value of a defined-benefit pension.
My best guess is that the MoD and Defence Council will limit its action to forfeiture of commission & pension for those who have taken this path to-date* with some tightening of legislation in the very near future. Anything else is just not worth the publicity and reputational damage.
(* There could be more specific criminal action for those who had particularly high security clearances, especially those read-in to US-owned compartments. The DoD (via DSS) owns that list and utilise the skills and reach of the FBI to investigate and indict.)