The Departure method of ident can only be done if you have an airborne time from the TWR.
The Tower will call you on the phone "XXYYZZ airborne 24"
If you see the target within 1nm of the runway you can identify it, if not, then another method of ident has to be used - squawk ident for example.
I think some of the regional airports in the UK use this method, but I am not sure if the busier London airports will call up with airborne times, which is why the "squawk ident" method is used
DF