At that price range, you're not going to do much better, and probably worse to extend the reach. There's the Sigma 70-300 f/4-f/5.6 APO for about £150; not going to give you stellar image quality, and very slow, but it does do 300mm. There's the Canon EF-S 55-250 IS, which is about £250.00, but isn't that much longer than your current lens, although it is stabilized.
The lens you probably really want is the
EF 70-300 f/4-f/5.6 IS USM but it's around £420.00.