Jim59:
Over in America it is quite normal for someone flying VFR to be granted clearance to fly through most terminal airspace, but as priority is (understandably) given to IFR aircraft and aircraft landing and departing the primary airport of which that airspace is designed to protect, you can also get refusals for Flight Following or transit during its peak time.
AFAIK there is no difference in priority whether an aircraft is commercial or non-commercially operated - everyone's safety and lives are worth the same - although the heavys will again understandably be given more attention.
Clearance or no clearance, operating inside, above or beneath the shelf of busy Class C and B airspace generally does require a mode-C transponder, which I think is very reasonable and certainly the safer option. It gives ATCers and other pilots equipped with traffic alert systems much less headache!