Maybe. PACE 1984 S19 suggests that if the constable thinks that there has been an offence, and that your phone is evidence relating to that offence, and that he has to seize it there and then to prevent its being concealed or destroyed, he can nick your phone. This is all based on a "reasonable grounds" test, i.e. the plod will make something up at a later date to justify their action if it's obvious that the constable has behaved unreasonably at the time.
Maybe. PACE 1984 S19 suggests that if the constable thinks that there has been an offence, and that your phone is evidence relating to that offence, and that he has to seize it there and then to prevent its being concealed or destroyed, he can nick your phone. This is all based on a "reasonable grounds" test, i.e. the plod will make something up at a later date to justify their action if it's obvious that the constable has behaved unreasonably at the time.