I used to work for GfK not on the research side but interviewers have to adhere to certain standards and the data they collect is bound by the Data Protection Act.

The interviewer should identify themselves and carry a badge with a registration number and say that the answers you give will be subject to the DPA.

Interviewers have to adhere to certain standards, google MRQSA or IQCS for details or contact GfK as the research industry works very hard to protect its name including spending a lot on training interviewers, perhaps this person needs retraining.