The phrase is 'remain outside controlled airspace'.
In the UK it is not listed as a readback item. CAP413 Chapter 2 Page 15 refers.
However CAP413 Chapter 6 Page 4 and Chapter 7 pages 2 gives examples of it as a readback item, but only in connection with a request to enter controlled airspace.
It is unfortunate that many pilots in the past have requested clearance into CAS and subsequently failed to remain outside CAS before clearance was forthcoming. It is my understanding that the practice of transmitting the phrase regardless of any such request was consequently introduced by nats as a so-called 'best practice'. In the absence of a request to enter CAS the phrase can only be a cautionary reminder and therefore is not an instruction requiring the response 'Wilco'. When operating outside CAS 'Roger' should suffice as a response to this gratuitous phrase.