I've always found that the plastic bit on its own only serves as a means of ID. From my understanding, to be regarded as a "complete" licence, the pink plastic bit AND the green paper bit need to be used/shown.
You ask any ideas? My idea is take both parts with you then you will be OK but are you saying you have lost the paper part of you licence? If you have lost it you would be advised to get a relacement fron the DVLA. This is because if you are given a producer by the police you must show both parts of your licence otherwise you can be summoned for failing to produce your licence.
The pink plastic card is only one part of a UK driving licence the green paper is the other part.