One question? Why do you want to join the CNC Police?
Undoubtedly they are very well financed and yes, have good training. However, operationally speaking they are not very active at all. In 2006, in all the sites they covered they had less than 100 recorded crimes. Additionally, although the marine escort group do get involved in transportation of escort to an from Japan, this is very rare and anyhow before you can get in you have to do your mandatory 2 year probation.
CNC also participate in SWAT competitions with US Law enforecement however, again, this only occurs once or twice a year. CNC also hold annual war game with Royal Marines as do the MOD police marine escort group however this is held on an annual basis only.
What I am trying to say is that you are not going to see or deal with a great deal, hence your operational experience will suffer.
To answer your second question...you can only transfer to CNC police or MOD police from Home Officer forces. If you join either CNC or MOD and then decided to transfer to say the MET or BTP then you will have to undertake your 2 year probationary period from scratch...this is partly because your warrant powers change slightly and partly because it is widely recognised that MOD and CNC do not offer officers enough incidents to succesfully deal with the wide array of incidents which occur everyday in cities.
Having said all this by either joining MOD or CNC police forces you obtain a
dv clearance and a good initial AFO training course. Depends where you want to go with it all really. Certain CNC officers also get sea survival training, naval cannon courses and other courses normally open only to military.
Hope this helps,
Ciao
Invictus13