This is very odd - British cars powered by DKW/Auto Union engines were pretty thin on the ground. According to Georgano the only model marketed in the UK as an Auto Union was the 1000SP, so that must be the source of the motor.
There were a few FJ cars with DKW engines, and John Aley ran a Sonderklasse saloon, but I can't think of anything else. Was the builder well-known in UK club racing circles?