It's very common in the US to pick up a IFR flight plan beginning at a point in space, if departing from a LZ without a instrument approach. Call the TRACON, get your clearance. They may ask you if you can maintain VFR to the first waypoint if the VFR transition is in controlled airspace and you may need to request special VFR. Picking up a pre-filed IFR flight plan in flight is also common, depending on how busy the sector controller is. I've always found ATC to be be extremely helpful and accommodating. How does this compare in the the UK?