If you search around you'll find a lot of discussions on this topic, this price point (esp for canon) has a range of options and there is a lot of debate as to which is the "best" item. In the end there isn't an overall "best", what there is is a range of slightly differently priced option which each have their own advantages and disadvantages. Good performing lenses in this price bracket all produce usable and good quality results so there are no 100% bad choices to be made.
A few to consider:
Canon 400mm f5.6 L
Canon EF 400mm f5.6L USM Lens from £1049.00, UK Specialist Price Comparison Site, Camera Price Buster just pushes the limit of your budget, but is the best optics you'll get at this price band. This lens will deliver the sharpest results and have about the fastest AF for this band of pricing. Downsides are that its a prime lens (no focal length changing) and that it lacks IS (image stabilizing). Otherwise its a very popular choice with bird photographers.
Canon 300mm f4 IS L -
Canon EF 300mm f4L IS USM Lens from £1099.00, UK Specialist Price Comparison Site, Camera Price Buster just pushing your limit again and 100mm shorter than the 400mm, however this lens gives you the advantage of the IS feature for helping when hand holding. In addition its optics are very good and this lens is often used alongside a 1.4 teleconverter which turns it into a 420mm f5.6 IS L lens. Optical performance just a touch behind the 400mm, but overall a very usable combo. The Combo itself will cost you more, of course, however you might be able to save and purchase the 1.4TC at a later date.
Canon 100-400mm f4-5.6 IS L
Canon EF 100-400mm f4.5-5.6L IS USM Lens from £1163.99, UK Specialist Price Comparison Site, Camera Price Buster - higher than your budget, but still within what most consider a reachable target without too much saving time; this is one of the superzooms in this price bracket and as such has a bit of a history to it. The superzooms (sigma or canon) all carry a little bit of "risk" with them; that risk is that they are very complex lenses and prone to having slightly more extreme tolerances of production. In general there are no problems, but these lenses can run the risk of being "soft" upon purchase - this is easily correctable, but will mean it has to be sent off to be serviced, furthermore if you don't send your camera body with the lens the servicing can be a bit of a lottery (since they don't have your body to calibrate to).
Don't let that discourage you, the 100-400mm has nearly 10 years worth of use behind it and its reliance has improved over those years - its probably the most reliable superzoom you can purchase and delivers very usable image quality (esp if you stop down from wide open to around f7.1/f8 when shooting at the long end).
Sigma sports a range of superzoom options - the 50-500mm OS
Sigma 50-500mm f4.5-6.3 DG OS HSM Lens from £1104.90, UK Specialist Price Comparison Site, Camera Price Buster and the 150-500mm OS
Sigma 150-500mm f/5-6.3 DG OS HSM Lens from £699.00, UK Specialist Price Comparison Site, Camera Price Buster . I honestly can't go too far into how these perform alongside the canon as I honestly am not 100% sure (plus I tend to mix them up a bit as to which is which). That said SCraig has already shown you what you can get with the 150-500mm.
I'd also recommend checking out
Juza Nature Photography - Articles who has a range of lens reviews on various options in this range and is worth reading.