They are the team that set up Screwfix down in Somerset. Then B&Q bought it out, moved the business to the Midlands and they were left twiddling their thumbs and / or redundant. So they set up again – as Toolstation.
In the early days (pre-1999), SF were catalogue-only and IIRC P&P was either free or pretty cheap – and next day. Looks as if they only went online just before they were bought out.
TS don't have the same breadth of range as SF (yet?) but often they are cheaper and / or they stock different brands / range. I notice that they are often literally right next door or across the road from SF(!), so I've often ended up calling in at both places. Got to be good to encourage them, so worth checking them out as well as SF when you are looking for stuff. Like SF, you can easily return unused or unsuitable goods and if you open a free account, the receipts are sent as emails so you can't lose them (works for SF, TS and B&Q).
Murray