Well thats the curious thing.... the older PPL texts say QNH digit by digit, but its the latest edition that says 'wun tousand' is OK by UK exemption!
Sounds like the latest edition of the RT book has lagged (a long way) behind the older CAP413 edition that suggested the use of 'wun tousand'.
Interesting that the current state of play is digit by digit. For the reasons above this one would be more sensible as 'wun tousand' wouldnt it. And there is a precedent in FL100 versus FL110 etc.
So it sounds like the rules say digit by digit but lots of common practice goes by the alternative.