flyinghigh,
basically I concur with the above replies.
However "flight Plan not required in UK class G", this requires a little thought. If one is expecting a procedural ILS then the airspace is likely to be controlled. Therefore to answer your first question "without informing anyone", you would have to obtain a clearance to enter the airspace with the approach facility.
Say for example the weather was closing in, then I would sugeest a call along the lines, "G-ABCD adverse weather, Request to continue IFR". The ATC would then have to increase to IFR separation requirements. Be ready for climb to MSA and for ATC to instruct to join at the IFR initial approach fix and respective altitude (typically the aid for the holding pattern), alternatively one may get vectors.
If you wanted a training ILS, then normally book in advance, no need for IFR plans.
flyme