You can leave the UP whenever you want, through a button in the foreign policy window.

I played one game as the Terrans where my first move was to leave the UP.  I was unable to have any trade routes with any civilisations.  They wouldn't send trade ships to me either but for some reason It was much easier to keep cordial relations with other races.

My good relations may have something to do with the economic starbases I built to help out my planets. I don't know if it also affects positively the other races in the range of the economic starbase.

I had a very hard time getting my economy in a good shape.  I went anomaly hunting to get some bc's at the beginning.  Aside from Mars I bought one planet right off the bat from one race.   I accidently triggered a war with the Korx when I was moving population from Earth to a colony I had colonized with a miner converted to a colony ship.     The Korx had a stronger military than I.  All I had were ships I'd traded from other civs.  I used a military starbase to help my invasion fleets.

I was barely able to defeat the Korx but this was a major setback to my economy.  The thing that brought it back was when enough time had passed in the game I got the research and economic treaty of every single major and minor race.  It also meant they were less likely to declare war on me.

I started allying with races that were becoming close to me.  My plan was to go for an diplomatic victory.  Every time a major race would conquer a planet though I'd try to buy that planet from them right way.  It was generally possible to do unless the planet was a civilisation capital.  Those extra planets gave me the influence I needed to flip a few planets.  I used spies to negate moral bonuses to facilitate the flips and I used transports to increase the population of newly acquired colonies from my most populous worlds (those that had maxed out their population).

I ended up winning via a influence victory although only one civilisation wasn't allied with me at the end.  (It was friendly and on its way though).  I was also 3 tech away from a technological victory.

Economy wise, it was like playing as a Thalan it was very hard to get it going.  Having a good diplomatic bonuses really helped.

Anyone know if you build an economy starbase on the path of another race's trade route if you will get some kind of bonus from doing that ?

  I have left the UP:  I was playing against only 1 race,  and a vote was passed to limit the number of starbase modules.  I think that leaving only prevents trade with other races and increases distrust.