If anyone does decide on this route, this website is useful.If you get to Paris, you could get a train to Hendaye? From there you can get the local metro to San Sebastián as well.
London to Spain by train | 2025 times, fares & tickets
Train times, fares & how to buy train tickets from London to Barcelona, Madrid, Seville, Malaga, Alicante by trainn. UK to Spain without flying! The train is an affordable environmentally-friendly alternative.
www.seat61.com