Some of the reasons to fly VFR on top.
To fly at an appropriate level for track.
To fly at suitable height for good coms and navaid reception
To keep clear of cloud, turbulance and icing.
To avoid flying in clag and granite.
To cross the UK coastline when sea mists invade.
To avoid miliary, restricted, nuclear zones etc etc etc.
To avoid local traffic around airfields.
To fly more safely.
To obey the French rules. Aircraft must be not less than 3300 ft above ground when overflying towns.
This assumes suitable climb and descent are available.
On the 300nm trip mentioned it would be a very good day when there was no indifferent weather anywhere en route.
The use of dead reconing for navigation would be the least preferred method.