is there any way to improve my relations with the pirates?
I notice that it never changes and I'm always at war and the diplomats so far cant do anything about it... so...
Is it possible?

Nope, Pirates by definition are at war with every established form of government. As long as you are working for England, your at war with the pirates. (It's in the game script aslo, nothing can be done.

you can never have a good relation with the pirates because each pirate ship/captian is against everyone else. including other pirates.

this is true. although you may not be able to improve relations with pirates, if you want to be like them, go to the dowesen pirates. they "will accept almost anyone into their ranks" says the dowesen tavern owner. and who says you always work for england? one time, i took over every english, french, and spanish establishment, and sided with portugal. go portugal!