Doing that trip in a small single VFR is easy (I suspect we're not talking Citation). Doing it in a fixed timescale, less so.
My last trip in that direction stopped at Jersey for cheap fuel, night stop and rendezvous with friends in La Rochelle, then to Valladolid over the top of a solid overcast over the Pyrenees and on to Portugal.
San Sebastian is a nice stopping point, but there are plenty of other options.
Be prepared to re-route or sit out for the weather (especially round the Pyrenees), but don't shy off. Those trips can become addictive.