AnFI, Bravo73's explanation for not using an NHS helipad is perfectly valid - Air Ambulances/HEMS don't shut-down on beaches below the HWM for precisely the same reason (and despite the supposedly-vanishingly-small chance of being caught out, there is history of it having happened).
As for using hospital landing sites for commercial purposes, I can see other limitations where business-folk only see possibilities. Fire cover at elevated hospital pads is provided by security/porter staff, neither of whom would be available for commercial tasking. Imagine your customers using the same express lift as the previous 'arrival', who may well have been inconsiderate enough to leave claret and other bodily fluids (solids?) around the walls, floors and ceiling. Imagine them then routeing through Resusc in A&E - 'cos that's where the express lift will be designed to go. Then imagine the delights of negotiating the standard UK NHS parking system to get to their limo...