alphaalpha makes a good point. It is the fact that whilst some units are clearly happy to help, some, particularly darn sarf seem to have an active policy of no ATSOCA. Everybody has their own impressions as to who the units concerned are, and Luton has already been mentioned.
There is more than the slight glimmer in many GA pilots' minds that the provision of service at certain units, and on certain shifts, has more to do with ATC politics than it has to do with the desire to provide a consistent response to aircraft operating in the vicinity of controlled airspace.
None of this is to deny the fact that many ATC units are badly stretched, and controllers are overworked. It does appear nonetheless that this is not the only reason lying behind the occasional denial of service, and the DAP is known to be looking into the matter, if recent circulars are anything to go by.
Hope this doesnt offend, but it does reflect common perception and experience.