Slough House is a department of MI-5.
"Let us be clear about this much at least: Slough House is not in Slough, nor is it a house..."
That is the opening line in the first recorded adventure of those who work in Slough House. If anyone really considers what they do there as work. Oh, they do things. They are given orders and tasks and eventually those deeds are usually completed but is any of it needed by anyone or is it busy-work? No one can really say, or if they can they certainly do not.
Slough House is the "dumping ground" for those members of MI-5 who have screwed up. What the infraction might be could easily vary, and does, but the destination for the "crime" is the same - get thee to Slough House. Drink too much or gamble too much or mess up an assignment or follow the wrong guy or let a protectee get dispatched or just really ticked off a higher-up and you become one of the 'slow horses' as the nickname goes for the denizens of this less than desirable posting. The poor fools who are directed to show up there every day did not do anything bad enough to get sacked outright but there is no doubt that it is hoped that with enough coaxing by menial tasks and lousy environs the bulb will come on above each of their heads that finding other employment is in order. So far, most have either not seen the light or ignored it or really weren't that bright in the first place.
Running the show at Slough House, or just herding the stock as some people consider it, is Jackson Lamb. Lamb is a most unusual man. He is considerably overweight and out of shape and yet he can, if needed, move with speed and grace that is a tad surprising and unsettling. His face could turn ten shades of red from exertion as he climbed a flight of stairs but it is a good chance you would not have heard him come up. He is very intelligent and capable when he wants to be but he most definitely would not go out of his way to make any of his charges know that. He is the king of all he surveys, or at least all that occupy the House and he can be quite a sarcastic tyrant when he wishes.
Lamb almost certainly does not like any of those forced to work at Slough House. It is a dead certainty that none of them like him.