i just wanted to ask did you have any trouble at border crossing into and out of germany.
What border crossing? As long as you have all your documents in case of a spot check by random Europeans, there's no borders and no hassle.
As for Poland - a British plated car is no more nor more less likely to be stolen here. In fact, I'd suggest that a British plated car is more hassle than it's worth to steal, as RHD vehicles can't be registered here.
Sticking to motorways is silly when it comes to fuel - you can save quite a bit of money just by going two minutes off the road.