And for the OP, if a navaid required by a SID is out of service, it is NOTAMed and aircraft who cannot accept the SID are advised to inform ATC prior to start. In 15 years at LHR, I can count of the fingers of one hand how many times an aircraft has requested vectors due to this.
We did a recent survey of operators, and all of those questioned are flying RNAV overlays on top of the conventional navigation SIDs.