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It seems that you do not need it. From the consular office at Lyon
1.B. Entradas por vía terrestre
Todas las personas de 12 años o más, procedentes de países o zonas de riesgo que lleguen a España por vía terrestre deberán disponer de alguna de las certificaciones que arriba se recogen (de vacunación o prueba diagnóstica o recuperación). Quedan exceptuados de este requisito:
That basically says that everybody over 12 crossing into Spain through land must have a valid CCD (minus a few exception for transportist, people living near the border and people who must cross the border for work regularly).
Also, point 2 is:
- Formulario de control sanitario (entradas por vía aérea o marítima)
which means "Sanitary Control Form (entry by air or sea)" which makes it rather clear that it does not appy to land travel.
A bit of a worrying point is that it tells that people
tiene que llevar consigo un certificado digital COVID emitido por un estado miembro de la Unión Europea (UE)*, que certifique alguno de los siguientes requisitos sanitarios:
must carry a COVID Digital Certificate issued by a member of the EU, which the UK isn't, but it looks more like an oversight than a refusal to accept UK's certificates.