Well I have been working a freq (which one escapes me - too many) and heard someone pass a flightplan over the radio. It took quite sometime and several transmissions, blocking the freq. The controller has to record all of the flight's details from endurance, equipment etc... He was not best pleased.
The flightplan should be sent to all addresses along the route, which you can add to, making sure all reqd LARS get a copy. Once you have sent a facility your fpl, a strip should be produced saying that you are IFR and become active once your fpl is active after your departure airfield has called to confirm your movement. If this is not possible, your first contact with the LARS will activate your fpl. I have used VFR fpl's to fly to Ireland and had to pass details and I am sure it is the same for non-airways fpl's, but it shouldn't if the correct addresses are included. The reason for the fpl is to keep controlers in the loop to your location and intentions, especially in bad wx or water crossings. But I am talking a 4hr flight in a SEP. I want all the bells and braces if the doink quits. When I am operating commercailly, it is always IFR and full radar / proc services all the way.
I agree, there is no perfect world and other traffic may be ahead in the queue, but the system is there to be used. If it doesn't work for you, ring up FAB or the local LARS and pay them a visit to clarify. Get PRO-ACTIVE and find an answer, not just saying it is a waste of time.
If you get a direct routing from Solent without a fpl you did pretty well. Normally they don't want to know GA as it is too busy there. It is bad enough taking an airliner in there.....
I think it depends on where you are as to the service you will receive.