"If it's too busy to issue a safe crossing clearance due to the "main customer" (the airports arrivals / departures), you aint going to get a transit clearance - period. No amount of complaining from anyone is going to change that."
This is the CAA's view:
There is no formal requirement for controllers to explain why a zone crossing clearance has been refused. Any such requirement would have significant R/T workload implications. It is considered preferable for refusals to be pursued after the event. Controlling authorities of new controlled airspace structures are now required to record refusals; pilots may in turn submit refusal reports in accordance with the procedure devised by Peter Skinner (AOPA/GASCo) or submit an occurrence report (CA1261).