As far as I can remember, Slough operate a similar system to Herschel - a certain number of places are allocated purely on rank order and the remainder on distance from the school, as long as the pass mark is achieved. Not sure of the exact split for Slough - as you say, it is 70 rank order and 50 distance from school for Herschel, so I imagine Slough will be similar, although they do have a much bigger intake - 180 pupils as opposed to 120. Langley give no priority to local pupils at all and offer all their places on rank order only. They claim that they have a "catchment area", but since this appears to stretch from somewhere around Ealing right over to Reading, I think they basically take the "pick of the crop" - ie purely the top scorers, regardless of where they live.
As far as cut-off marks go, it does vary from year to year. Although Herschel's was 116 last year, the initial offers only went out to pupils scoring 117 or above (unless you got one of the "catchment places", of course). They dropped their offer to 116 very quickly though, I believe. Not sure what the cut-offs for the other 2 were, but I do have it on very good authority that everyone living in Slough who achieved 111 or above did receive an offer of a place from SGS (unless they already had an offer from a school placed higher on their CAF form).
I guess it's a case of "wait and see" for this year's offers?!