Best Fully Regulated Sites

I understand several US sites are fully regulated but unfortunately the player pool is limited. What are some other safe and reliable sites backed by a reputable regulatory authority? Are the various UK ones trustworthy?

The entities that offer gambling for UK residents need to obtain the licence from Gambling Commission. The UK poker websites usually display the link to GC on the main page. These that do so are trustworthy. The examples are pokerstars, partypoker, betfair and others.
Some poker rooms have no uk gambling licence , but they usually are closed for people from UK. They are usually located somewhere in Europe and may be less trustworthy.