In which case, surely, one needs to file an IFR flight plan, via Brussells, 3 hours before the flight.
It is true that in order to fly an IFR flight in controlled airspace, Brussels needs to get a flight plan. 3 hours is a little extreme, and most flight plans can be turned around inside an hour if you have take the trouble to pre-validate the route.
However, if you are implying that having a flight plan in the system will in some way ease your clearance into controlled airspace, this is only partially true. There is a considerable gap between filing a flight plan, and finally receiving a squawk, frequency and clearance to blast into controlled airspace. Your flightplan will not necessarily be sent to every conceivable class D provider that you might wish to use to organise you clearance, and therefore you will be treated with caution if you call xyz Approach up unannounced looking for asssitance. This is precisely the point that AlphaAlpha makes.
As a result, the normaly way of getting your clearance, having departed from an uncontrolled field, is to call London Information telling them your registration, position, time of departure and requirements, and they will activate your flightplan and liase with an appropriate ATC unit to arrange your clearance. In due course, they will relay a squawk and frequency and hand you to your airways controller (or as appropriate).
2D