AlanM
I'll think you'll find that there are two possible solutions in use at t'other end of the room which can be used at City
At Luton they use Standard Departure Routes; these are published in the AIP and are specifically designed for aircraft that depart from an airport inside CAS but which are then leaving CAS. The SDR includes routing and level instructions which comply with the minimum noise routes for Luton (in effect they mirror the SID route) but climb is restricted to 2400 feet or another level which keeps the departure beneath the LTMA. Departures from Luton to EGWU, EGLC and EGTF use these if they aren't going into the LTMA. Because the routes are published and are specifically for aircraft that intend to route outside CAS there is little room for confusion.
At Stansted there are two SIDs which go out through BKY which stop initially at the same level as an airways departure but the routing after BKY is different, these departures leave CAS north of BKY. If however the departure is going on a track which doesn't go through BKY, towards EGMC for example, they are given a SID but with a restricted altitude.
In my experience the Luton solution is better because there is less opportunity for confusion and crews can brief the departure at their leisure rather than get a convoluted restricted SID route.