According to the police, speed camera warning signs are not required
However, this is not entirely consistent with the Protection of Freedoms Act (PoFA 2012) and it may be that you will need to have signage for the roadside unit as it contains a public facing camera and therefore falls within the PoFA definition of "Surveillance Camera". According to the guidelines, camera surveillance in public requires signage (and that is only fair given its prolific use when recording and monitoring law-abiding citizens; AutoSpeedWatch only records where offences have occurred).
On our understanding of UK Law
we recommend you use our information signs, even though an image is only captured, and the LPD assistant tool used, only after a speed offence is detected. As always, we suggest it's worth involving the local police authority, local councils and highways agency from the start.