Budget is the key really and I am assuming under £500 another consideration is are you happy to change lenses when out and about.
If you go down the zoom route and are happy to change lenses then under £500 there are two main options the 70-200 F4L or if you need more reach at the high end then the Tamron AF 70-300mm f/4.0-5.6 SP Di VC is a lovely lens. If you want more reach then the lenses will go up in price and weight. I would steer clear from the Canon 70-300 EF IS though unless of course you budget stretches to the new L version
If you don't want to change lenses then you may want to consider a super zoom two lenses spring to mind here the Tamron 18-270mm f3.5-6.3 Di LL VC and the Sigma 18-250mm F3.5-6.3 DC HSM the Tamron has slightly more reach but I believe the Sigma is marginly better optically. Remember with super zooms are a compromise so image quality will not be quite as good as the 70-300 or 70-200 also you will tend to suffer more chromatic abertions and barrel distortion but these can be corrected in PP anyway.