I live in MK too and have run several IS models, I had the 220d new in 2006, an IS250 f sport in 2010 and just changed to a new IS250 Premier. Even running on the dual carriageways daily I can't get past 29 mpg yet. I've only done one proper motorway run, and that didn't help due to the fact that it was the first chance to really open it up. I'd expect that with a light right foot cruising on a clear motorway, I'd get into the low 30s per gallon.