As an example I can more easely get a crossing clearance through Birmingham IFR than if I ask for a VFR crossing. At least IFR i can be coordinated with other IFR traffic VFR i will either be told to remain clear or given a crossing only when traffic allows.
An interesting point - I wonder what the controllers amoung us think?
I can think of occasions when crossing CAS at height and on a long route the controllers seem much more inclined to give a clearance on the occasions they are turning other traffic away. However has this more to do with your altitude and their realisation that if you are on route from A to B it is a nice courtesy not to make you divert around CAS. Moreover because usually there is so much less "high" traffic clearing you through presents them with less problems.
I suppose my point therefore is that do you more easily get a clearance because of what you are doing rather than becasue you are IFR?