My understanding is that it's not illegal
I think you will find that the Mobile Telephone Company's Ofcom Licence to operate prohibits aeronautical operation in which case you would be in breach of the licensing conditions! This was true of early Cellphone Operator Licences; I am not sure if it still is.
Cellphone antennae are designed with a very low angle of radiation to reduce power and minimise their use at altitude, where in theory they are not needed.