You should try stationing an army in a fort at your border with Poland. If they declare war, they still have to get past the fort. A stack of troops would be nice. They can even be cheap infantry of some sort. I have found that fighting fort battles are a great way to wittle other armies down. Keep the fort wooden as they are easier to defend.

Most of the time the armies next to your borders are militia anyway. In my English campaign, the Spanish had 8 stacks around Gilbralter. 5 of the 8 were almost all militia. The only arty he ever had consisted of Demis that were placed too far away to be any problem.

As others have stated, even if you have to pay to trade, I set up as many trading partners as I could handle on the first and second turn. Some of the AI require money along with the trade agreement.

I found it odd that UP goes to war with GB. Probably what happened is they declared war on the Thirteen Colonies. That happened to me twice playing as GB. As GB, the first thing (the second game) I did was cancel my alliance with them. They remind me of Milan in MTW2....already at war with two major powers and then decide; "Let's see who else we can attack!".