I think the way it is intended to work is something like this:
You must depart in VMC on a VFR flight plan. You may not enter cloud until having reached whatever height is specified in the rating. If for example this ends up as being lets say 1,500 feet you will proceed outbound from the airport on a VFR clearance and if lets say the cloudbase is between 1,300 and 1,700 feet in the vicinity you will arrange you route with the controllers so that you can find an area where you can climb to your en route height without entering cloud until you are at least at 1,500 feet.
You will presumably have declared in your flight plan the point at which you expect your flight to become IFR and also the point at which you would like to join a lower airway. If you have been displaced by the cloud base during the climb presumably you will now ask to join somewhere else on the route or will head back to your planned join point and tell the controller that due to the cloudbase unfortunately you will be joining at a later time than planned.
The Controller may or may not now give you the clearance you requested. If he doesnt you will continue in solid IMC rather than achieving the expected VMC on top and generally increase both your work load and cause a satisfying amount of chaos in the system generally as you seek to renegotiate your join. Your departure planning will have long since gone out of the window and you will be working hard potentially in IMC repositioning the aircraft and replanning the flight.
If you are lucky you might eventually secure a join, if you are not, its enroute for you remaining outside of CAS negotiating every clearance and if you are very unlucky IMC the whole way. Enjoy.
Of course when you arrive you will have to explain to the Controller that you need to negotiate a VFR arrival and will need a descent somewhere suitable in order to achieve this. Its down into the soup again for you. Where? Where indeed, who knows, because the Controller sure doesnt want you making up a descent in his backyard. Down you go somewhere, but as long as it is not in my patch. Of course you dont really know when, or should I say, if you are going to become visual within the limits of your rating. I can hear you curse as you dodge between clouds and gliders that someone should have told the clouds they must form up into a uniform base. If you are very unlucky the base will have changed since your departure three hours ago from the South of France - someone should have told those pesky clouds that they really should read and abide by the forecast. I know you are an astute pilot so you got the forecast before crossing the channel so you would be ready to divert into northern France - what a shame the base is a few hundred feet lower over the north of France - so its a diversion for you my boy. Of course you are getting pretty good at this replanning lark now, so its out with the map, on with the radio and some fun and games with the Contollers getting them doing some work to find the weather at your alternates - ah well they havent got anything better to do have they?
Easy stuff this IR flying I remember the days I flew on an EIR - now that was hard work.
