I took it to mean 'are you allowed to undertake a lorry that it is to your right' (assuming the lorry was going along in the inside lane, and you had gone onto an exit slip road to the lorry's left?) I don't know the answer though - lorries can go so slow, especially on hills. Some exit slip raods are really long, otherwise I'd say that people should be slowing down anyway on them. Will be interested to know the answer, as I have 'undertaken' in that situation in the past.