Best telephoto for a canon?

I've got a budget of about £600 pounds and am looking for a good quality telephoto that is around 300mm or more if possible. I don't really want to have to use a Nikon to Canon adapter so that removes a few options. Any suggestions greatly received.

The 70-300mm IS USM lens

should be in the £400-500 range

I like the Canon 300 mm f/2.8 but your budget is a little low. That lovely lens cost closer to £6, 300 (10x your budget).

If you are shooting outside during the day, the Canon 300 mm f/4 may be a closer fit to your budget of £600, but used they cost about £850

For £674 (€ 815) you have the Sigma APO 150-500mm F5-6.3 DG OS HSM Canon
(No adapter required)
http://www.foka.nl/site/sigma-150500-mm-f563-os-apo-canon-p-5687.html?vergelijk=true

The Tamron 70-300mm f4-5.6 Di VC USD is actually better than the Canon 70-300mm f4-5.6 IS (which is of an ancient and dated design). I would not recommend the any of the non-L Canon 70/75-300mm lenses.

Canon's 400mm f5.6L USM is also quite inexpensive second hand, around the £500 mark, it's probably the best option you could go for.

Canon's 300mm f4L USM (non IS) is less than the 400mm, however it's not very common, being replaced by the 300mm f4L IS USM quite early on, and the IS version is expensive.