I'm guessing it's cuz the 2.5 requires more gas to get adequate performance. It's perfectly adequate for tooling along. However, many 325 owners live in areas that are pretty wealthy and thus are surrounded by other luxury cars. Luxury cars are getting some REALLY powerful engines nowadays and I suspect 325 owners have to gas it more just to keep up, thus causing worse gas mileage than the more powerful 330 owners. If you're in an area with tons of 4 bangers (like I suspect you are in jolly ol England), then you prolly won't have to gas it so much and will get about the same gas mileage as the typical 330 owner.
Tranny (automatic only) really only had problems for 3 series cars built towards the end of the 03 model year and all of 04 pretty much. If you're looking at 00/01, you're fine with either tranny.
I will suggest you get the 3.0L if at all possible. I dunno how the insurance/taxes affect cars at the 3.0L displacement level, but performance wise, that's the best car. The 325 is close enough to the 328 that the 328 might not be worth the premium. The 330 is worth it tho. Least in America where the insurance is barely different.