So, after having looked on the internet for all kinds of possibilities: plain, unadorned, unpainted, undecorated, etc. I have looked on Homebase, B&Q and Wickes and found no real door that is the equivalent of a 'schilderdeur'. This kind of door is a standard, single panel door, undpainted and without mouldings or notable hardware. The buyer still has to sand and paint it.

The English being incredibly traditional, shops just mainly sell panelled doors, though there are rare unmoulded doors, but they are just labelled 'door' and then what they are painted or veneered in, which is not really what you want.

So, the above is my proposal. They'll certainly know what the client means, seeing that. If it is painted standard grey, for example, then put 'grey' in place of 'unpainted'.