You can file a "Mixed" flight plan if you expect to enter IFR along the way. Codes are Y for IFR first and Z for VFR first; include in the FPL routing the points where you'll change rules.
That's fine,and you can fly in IMC on the "IFR" sectors with an IMC rating as long as you are in UK airspace and not in Class A. Talk to the relevant ATC unit as required.
Once you reach the French (or any non-UK) FIR boundary, you have to be VFR and in VMC unless you have an IR.
If you filed VFR and find yourself getting into IMC, and if you're in airspace that requires it, then you need to call the controlling ATC unit and tell them you're IFR in IMC. That means all airspace except class G, if my memory is working today.