The three that I'm aware of nationally in the UK are Calumet (expensive), Lenses for Hire and Hire a Camera (both reasonable, but read through the terms and conditions and insurance information). I know of people who have used Lenses for Hire and have had a good experience and I've heard similar reports online regarding Hire a Camera. I know that Lenses for Hire also rent out TSE lenses, whereas Hire a Camera has a smaller range of lenses. There are probably more local sellers that also rent out, such as Bristol Cameras or Clifton Cameras (I have a feeling one of them does, but I can't remember which one).