Longbow55, it might be worth pointing out to your oh-so-helpful tower that even if they can't get you the actual "join the airway" clearance they should be able to get you a squawk and a frequency to contact after take off. The nice thing about this, other than potentially avoiding having to join a queue to speak to London Information, is that you find out if your flight plan is "in the system" before you leave the ground, rather than at a thousand feet in IMC outside CAS, when you've probably got better things to think about.
In other words, as Chilli sort of says, there is a distinction to be made between the full airways joining clearance and an initial clearance of the type that Chilli points out.