My understanding is that ATC can only refuse to issue a VFR dep clearance if the weather is below the minima IF they are in a Control Zone. (remember the halcyon days at the College of ATC saying "unable to issue a VFR clearance what are your intentions?")
With regards to a zone transit, again I believe that it is for the pilot to determine whether or not they can fly VFR.
Vfrflyer - Have Cranfield filed on you then???