From CAP 493 (my bold)
Transfer communication At (time or significant point)/When passing/leaving/ reaching (level):
contact (ATSU callsign) (frequency).
Contact (ATCU callsign) (frequency).
Stand by for (ATSU callsign) (frequency).*
Monitor (ATCU callsign) (frequency).*
*An aircraft may be instructed to 'Standby' on a frequency when it is
intended that the ATS unit will initiate communications soon and to
'Monitor' a frequency when information is being broadcast thereon.
Basic Service available from (agency) (frequency).