You must obtain a crossing / joining clearance to cross / join controlled airspace - PERIOD!!
I think ATCOs could be slightly more explicit in an effort to assist the eductaion of pilots to this end.
The issue is most pronounced on joining the airways system from outside controlled airspace, where routings are issued with a clearance implied. For example, on coming down from Carlisle:
"G-ABCD route MCT, HON for Cambridge"
was what I got from Manch by way of clearance. Similar often happens outbound from Cambridge to the south, with no explicit clearance given, but instructions that clearly require me to enter controlled airspace.
I don't think it's a big deal, but if ATCOs got into to the same trouble every time that they failed to issue an explicit clearance for an aircraft they expected to enter controlled airspace as pilots do if they enter without a clearance, the world would be a different place.