Are you saying that it goes to Eurocontrol only if some waypoints lie inside Class A? How is the interesection determined?
I think it is possible to underestimate how much of this is down to the judgement and experience of the individual processing your flightplan.
If you file a flight plan which the person doing the processing thinks is ambiguous, you will probably be asked whether or not it was your intention to enter controlled airspace. If so, the person doing the filing will probably pull you up on your route and ask you to provide better particulars about your intended point of entry and exit from controlled airspace and the standard route you intend to take between those points.
More likely though, your plan will simply be filed as non-Eurocontrol, complete with stupid flight level and non-standard routing and if you intend to enter class A enroute, it will be up to you to negotiate a clearance enroute.
Don't forget that whether IFR or VFR, a basic non-Eurocontrol Flightplan within the UK is of very little practical value to anybody save SAR - we don't even follow ICAO practice and formally "close" flightplans in the UK.
Are you saying that all IFR FPs with waypoints outside the UK FIR go to Eurocontrol automatically?
That is not what I wrote, but in effect, this will normally be the case. In the vast majority of Eurcontrol countires, you cannot file IFR without being part of "the system", and holding an instrument rating. You will therefore be required to file along standard routings because your plan will pass through Eurocontrol's clutches. Britain is relatively unique in allowing uncontrolled IFR flights "outside the system", so it is similarly unique in requiring any judgement in interpreting pseudo-IFR flightplans.