The bus route may have changed. But the last time I caught the bus from T5 to Slough (which was the number 81), it pulled into the bus station right next to the station (although other side of the track to the HI Express and I expect you can't use the station bridge without a tciket).
The 81 does not go to Heathrow Central or Terminal 5. It runs from Hounslow via the Bath Road to Slough bus station avoiding the airport. It is a
TfL» route (red bus) run by London United. T5 to Slough would probably be a blue bus run by First.