This car was apparently known as "The Red Monster".
It was not specifically a prototype for the Lagonda Rapide as it was actually built in 1949/50 and registered in 1951, whilst the new Rapide didn't appear until 1961. Nevertheless there are similarities in their appearance; it could be said that this car looked quite ahead of its time for a 1949/50 UK car whilst the Rapide, when it did appear, looked old-fashioned for a 1961 car...