The Sigma 70-300mm APO is the pick of the bunch when it comes to cheap telephotos. However, there's always more than one choice, depending on what use you expect from it...
Canon offer the EF 28-200mm f3.5-5.6 USM for ~£253, the EF 55-200mm f4.5-5.6 USM II for ~£153. And the fabled EF-S 55-250mm F4-5.6 IS for about £220 Sigma do a 28-300mm f/3.5-6.3 lens for under £200, give you a walkround lens you'd never take off. They also do an 18-200mm f/3.5-6.3 DC for ~£146, and an Optically Stabilised version for £265 (sell your kit lens, make up the difference?). Tamron also have the 18-250mm F/3.5-6.3 Di ll LD Aspherical (IF) Macro for £289, or the 28-300mm XR Di LD Aspherical Macro IF f3.5-6.3 for £229. They also have the 70-300mm f4-5.6 Di LD Macro, but to be honest, the Sigma version trumps it.
Would 200mm be enough for you, or do you think you'd need the extra reach the 300mm zooms would give? The walkround lenses (18/28mm to 200/300mm) would be an all-in-one solution, the 55/70 to 250/300mm would be a second lens you'd change as required...
Of the Sigma/Tamron lenses that I've owned, the 28-200mm was a fairly decent walkround lens, covered 95% of everything I shot at the time. The Tamron 70-300mm suffered from a fair bit of chromatic aberration at the long end, and the Sigma 70-300mm APO was just fantastic for the price.