I find it is worth telephoning ATC the day before and asking their advice if your departure is near one of the major airfields. I have always found them extremely helpful on the telephone. Some of the lower flight levels seem to be more or less permanantly assigned to the radar controllers at nearby airfields so they can often clear you quickly into class A with a suitable limit while they get your clearance from London (or whichever control).
Its easy to ask them on the telephone what the simplest proceedure is going to be for a particular flight.
They won't always get a copy of your flight plan but you can add their address at the top of CA48 and then they certainly will. This helps too because you are not then such a surprise!