The State of Massachusetts will not accept a post doc in the UK. Nonetheless, all of the credentials as a PsyD or PhD do cross over in the UK. There are several tiers and if you completed your doctorate (minus EPPP and post doc) you can practice in the UK as a psychologist. You have to go through the British Society of Psychologists, that is equivalent to the APA and they take approximately 16 weeks to process your application.
As a caveat, I am a British citizen and you do have to have citizenship to do this or a company that would sponsor your work visa.