The Sennheiser HD25 II are a great portable, but I cannot directly compare them to the K240 because I haven't thoroughly auditioned a K240. I suggest that you go try an HD25 somewhere and see what you think, but they're pretty much the standard for portables.
HD25-1 II - not the SP.
I agree with the suggestion. Also, M50s are good for portable use, although a little bigger than the senns.. Not sure if these headphones sound anything alike the AKG. AKG K518 is a little bassier supra-aural one and AKG K450 is a pretty small headphone. My wife adores it, although I don't have too many good words about it.. YMMV of course.
Just come across this review http://www.headfonia.com/closed-cans-shootout-m-50-esw-9-t50p-hd25-1-beats-studio-srh-840-srh-750dj-k181dj-and-dj1pro/ and I will be having a look out for some of these this weekend.
Anyone know any shops in the London area that will have most of these on display to try? Some shops like HMV and the like just have Beats, Skullcandy etc on display.