According to PhotoPlus magazine, the Tamron SP AF 70-300mm f/4-5.6 Di VC USD is the best budget telephoto zoom to buy, which should cost around £300. On a tighter budget they suggest the Canon EF-S 55-250mm f/4-5.6 IS II.
I have the original Canon EF-S 55-250mm f/4-5.6 IS lens and although I get decent shots would recommend getting the Tamron for the better build quality. The image quality, autofocus and image stabilisation will also be better.
I tend to buy Hoya Pro1 Digital UV filters for my lenses.