I'll let II give the definitive answer - but it is something to do with where they could be confused with a red signal next to a running line, they are made to be white instead.
Buffer Stop lamps are red except where confusion may result in them being seen out of context by an approaching train on an adjacent line, when they will be white. This usually happens on curves.
Thanks, both contexts seem to apply at Slough. Confusion with Red running signal and a curve.