My first thought would be the Nikon 70-300VR. Good mix of quality, reach and price.
Yes you can get cheaper 70-300's but the price refelects the quality.
You could get an old AF-D 300mm f/4 prime and 1.4 TC, but you wouldnt have autofocus.
Other options are a Sigma 120-400mm OS, or 50-500mm.
I think you'll need to spend a minimum of £300 to get anything half decent.