I was there on official business in this instance. Having approached the staff in order to get to the Duty Managers office because I wasnt prepared to buy a ticket, I was refused entry because the Duty Manager was not around, and to come back 2 hours later! If I made any attempt to photograph the station I would be detained for the BTP to deal with, "Terrorism and all that" was the comment. Not a chance as this was a week day and I had to go to work.
The next day at the same time I tried again, this time with the Duty Manager around. However again entry was refused because they didnt have any paper work and they did not permit any such photography (personal, commercial, educational, referencial or legal) at Slough. Great another trip wasted.
On the third day I had made the necessary arrangements with First Great Westerns Corporate people, and had the paper work in hand on my arrival. The staff duelly accepted and I proceeded to the Duty Manager for that day. Yet ironically this guy didnt even care! He certainly seemed alot more senior than the staff of the previous days, and his position was that if I was not attempting to get anywhere that the public are not permitted then I was quite welcome to take pictures, and thanked me for making the effort to get official permission from FGW! Sheesh!
However despite all this comedy just to take some pics, having ok'd the session with the Duty Manager, having notified several security staff of my presence, I was STILL stopped and questioned 4 times in 45 minutes! Twice on Platform 2, once on Platform 4 and again on Platform 5, by dispatch staff.
So hence the reason for advising against it, purely for the hassle you can expect. Although because its busy at work, and I didnt want to loose the time, this was all taking place between 8 and 9am. Which COULD account for the staff being on edge.