You have yourself a very good camera, so why cripple yourself with a budget lens? Particularly as you state that image quality is important.
That camera pack 18 million pixels onto an APS-C sensor and what the camera makers and photographers are finding is that you need the best glass you can put in front of the sensor to take advantage of all those pixels. (part of the reason why we are seeing upgraded lenses being announced/shipped - e.g., Canon 70-200 mm f/2.8L IS II)
Having gone down this same road before I recommend you wait and save up some extra money and purchase a better quality lens. Otherwise you spend 300 quid now, realize in a year or two you want better image quality and then go out and buy a better lens anyhow at a higher price.
Give a lens such as the Canon 70-200 mm f/4L a consideration for your 7D. I saw it advertised in the back of one of the British mags I subscribe to for about 500 quid, so 200 quid over your stated budget, but the image quality you will get, as well as better focusing speed/accuracy and a host of other benefits, will be worth it in the long run.
The other thing to keep in mind, is these digital SLR cameras are essentially disposable. It is the lenses you will continue to use from camera to camera so invest in the best glass that you can.
Here are links to some review sites I frequent, that talk about lenses and that you might find helpful:
http://www.the-digital-picture.com/Reviews/
http://www.fredmiranda.com/
http://www.photozone.de
Best of luck in your decision,