nouseforaname -- from my own experience:
First let's assume you have filed a flight plan and it has been activated. Yes, I know it's theoretically possible to file a full flighplan when in flight, but it's messy and makes your life very hard indeed.
Second, if you are flying from an ATC/FIS field and aiming to join CAS shortly after departure, your departure ATSU should get your clearance for you -- even if it is only 'remain clear of controlled airspace, squawk xxxx and contact 123.45 when instructed.'
If you are departing from an ATC/FIS field, but your airspace join point is many miles away, an en-route request for a clearance via London Info will usually be successful if you call the early enough and with a good estimate for your join point. However, don't try this on a sunny weekend in summer. Success will often depend on whether the airways controller has prior knowledge of your flight, ie has a computer produced a strip for him.
I suspect you are departing from an uncontrolled field. In this case, you need the help of a local ATSU. This is best arranged by telephone before departure. You're really asking a favour so be nice. Tell them what you plan to do, where you want the join and when you will depart. Ideally send them a copy of your flight plan. Then freecall them after departure with your estimate for your join point and (usually) all will be well.
I fly from an uncontrolled field, so have had this problem from time to time. The above paragraph is my solution.
AA.