The normal procedure for an airways IFR departure from an uncontrolled field, does not vary much.
If the field is A/G or FISO controlled the clearance will often sound like this :
"London Control instructs N33NW to remain outside controlled airspace, squawk 1234, and to contact xxx.xx when ready"
Where the field is a farm strip, you could telephone for some sort of clearance, but it is more common to take-off and use either London Information or a local LARS/Radar unit to negotiate your entry into controlled airspace.
Getting a clearance straight into controlled airspace from the ground under either set of circumstances is vanishingly rare because of the airspace that such a clearance would tie up. This is despite it being common practice in the US with its somewhat different airspace structure.
Whether you deem yourself to be IFR or VFR before receiving your clearance into controlled airspace is a decision you take based primarily on the prevailing weather conditions.
2D