I know I'll probably just get shouted at for poking my nose into Motors.

But how about a Skoda Superb estate? You can buy one spanking new for just over £20k, at top Elegance spec with full heated electric leather, built-in voice activated SatNav and full bluetooth pack, automatic everything, adaptive xenon lighting, all the usual toys etc etc.
If you went second hand, you'd get a nice DSG automatic one basically a couple of months old for well under budget. They return around 55mpg to 60mpg
combined (depending on engine choice) and are very, very comfortable. If you can live without leather, you're looking at 15k or so for an auto fully loaded luxobarge estate.
Yes I know it's not a BMW but it's solidly built and supremely comfortable. They also hold their value as well as any Mercedes. The interior space is bigger than a Mercedes S Class, and it takes a BMW 7 series
long wheel base to match it for size. Just about 15k cheaper... Just a thought.