Ok - quick definition first:
"Flight Plan - details of a flight or portion of a flight submitted to enable issuance of an ATC clearance".
IFR/IMC in class 'G' airspace is not subject to an ATC clearance, therefore no flight plan is required. There was nothing wrong at all in the plan that you filed. You just changed your mind part way through and resumed VFR later
If you were to file an off airways IFR plan then you would never be instructed to enter class 'A' anyway - as you wouldn't have planned to enter. In addition none of the ATC units on your route would try to get you to enter class 'A' because they wouldn't even have a copy of your plan - that would only go to your departure airfield, destination airfield and your alternate maybe.
If you intended all of your flight in UK airspace, including the departure, to be conducted IFR, then you should file a plan. But again, this is more relevant if your flight begins or ends at an airfield with a proper ATC unit.
CM