Woss goin on..? asks a number of related but distinct questions. First the legality issues:
1: Is this departure legal under the VFR rules
To enter IMC requires (by definition) the flight to be conducted, at that point, under IFR. There is no reason why a flight should not change from being a VFR flight to an IFR flight at any point, from moments after take-off to moments before landing.
In controlled airspace, a clearance is required for IFR flight. Therefore the scenario described would be illegal, unless approved in advance by ATC in which case it would almost certainly be pointless.
Outside controlled airspace, a clearance is not required for IFR flight. Thus it would be perfectly legal to switch from VFR to IFR when required. The only wrinkle might be if the airport of departure had ATC, in which case the controller has the right to instruct the aircraft to maintain VFR within the ATZ. In those circumstances it would be necessary for the pilot to remain in VMC until outside the ATZ.
Now for the is-it-sensible? issue:
do you consider it sensible to depart in these conditions, fly IMC with no form of radar cover, and no way of avoiding other aircraft which may be in your path?
There seems to be a level of confusion about aircraft separation implicit in the question.
On the assumption that the flight is being conducted outside controlled airspace, there is no reason to believe that the flight would be better separated from other aircraft simply by virtue of being IFR rather than VFR. If radar is available, a radar service may (and probably should) be offered to VFR flights as well as IFR flights. If radar is not available, the IFR flight is separated only from other participating IFR flights. If there are none, or the pilot of the departing flight is aware of the other IFR flights known to ATC, then that minimal separtion service is no better than what the pilot can achieve with common sense.
Then the motive:
what reasons would a pilot choose for not departing IFR? Is there a worry about route charges or something similar?
I would imagine that might be the issue. Charges are applied to aircraft between 2 and 5.7 tons if any part of the flight is conducted under IFR, regardless of the service provided. If a flight departs VFR, it is less likely to find its way onto Eurocontrol's system.
Finally
do you consider these rules are clear, and understood fully by pilots or are they unduly confusing?
The complexity of regulations often go with the flexibility they offer. In this case a great deal of flexibility is offered compared with, for example, the German practice of simply banning IFR outside controlled airspace. If the complexity of the UK system troubles anyone, there is no reason why they should not simply decline to fly IFR outside controlled airspace.