It might appear that it's legal so long as the battery isn't with you as luggage. With it, an argument could be made that it's capable of being powered by you. Without the battery with you, it appears to be no different from a bike carrying a motor as luggage.
However, there may be complications. If it's a manufacturer supplied e-bike, it may have been certified as a different EU class e-bike under EN15194, and that might be used as a basis for prosecution. That could apply equally to an e-bike label if fitted. The fact that it was only being pedalled might carry no weight since e-bikes are also pedal cycles, albeit with optional assistance.
A possible parallel is a case of someone pushing a motor vehicle of any sort that has no petrol. They are still in charge of a motor vehicle and subject to all the laws relating to that. On that basis I think a prosecution may well be possible, since there are some laws that may apply, mainly the 1983 EAPC regulations and the 1988 Road Traffic Act.