Off watch, my take would be that as the UK AIP is for pilots, it is advising them that normally they would plan IFR unless there are exceptional circumstances. However the AIP is not precluding such an operation because firstly it is not unsafe and secondly because it is impossible for the CAA to second guess every possible special circumstance that might occur. Naturally the controller does not need to ask the question because we are not policemen only separators.