You have a few options.
1. First-Gen Minolta AF
Lenses. these are all very good and can be had fairly inexpensively. Minolta 70-210 f/4 is the staple. Good image quality, OK AF
. Its bigger brother is the Minolta 75-300. It looks just like the 70-210 f/4 except the zoom extends. There is also the smaller 100-200 f/4.5, only problem with that is minimum focusing distance. they all have about hte same kind of performance. You could probably find them for 150 GBP or less.
2. Sony lenses. The Sony 55-200 is a kit lens type telephoto but the optical performance is actually fairly decent. It won't win any beauty prizes though. The 75-300 is a skip, IMO. Then you start getting into the more expensive territory, things like the 70-300G and 70-400G. These are excellent lenses with SSM (ultrasonic) motors.
3. Later Minolta lenses. 100-300 APO, 100-400 APO only available second hand but are a good compromise as they are priced inbetween the 70-300G and the old beercan but give image quality somewhere inbetween.
4. third party. I'd suggest avoiding the tamron/sigma 70-300s and look at something more like the Tamron/Sigma 70-200 f/2.8s> These will cost as much as the 70-300G but they trade range for a faster aperture. There's also options like the Sigma 50-500 or tamron 200-500 but again that starts getting up there in price.
I suggest going to dyxum.com and reading lens reviews and looking at sample images. Good luck!