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).
Lens Pimp are very good, rented the EF-S 10-22mm & 24-70mm L lens from them, perfectly fine, no hassle - though their communication leaves a little to be desired but they do get the lenses to you nice and early so you get an extra few days on your rental (at least that is what i've experienced, don't quote me on that or expect it to happen! )
There's a company called Lens Fettish. Think it's a very small outfit (a slightly odd website). I have no idea if they are any good but I recently purchased an ex rental L lens from them on ebay (only a year old according to the date code) and it's a fab lens. So I reckon they regularly update their stock and look after it, though I have a sneaking suspicion that they may only have one of each lens.
I wanted to rent recently but was put off by cost (delivery charges), I looked at http://www.hirental.co.uk/ and Calumet. Just checked back at hirental and I’m wondering if they are still trading though. I’m off to check out lens pimp, didn’t know about them thanks ;D
I've used lenses for hire a number of times, a 35mm f1.4 L and a 400mm f2.8 L IS. Both lenses were well used, but optically in perfect condition. The service from the company was excellent and highly recomended.