Ideally you need to be looking at speakers with a 90db efficiency rating or higher. At shows I have to say I have not heard what i would consider a bad speaker from the firm Triangle ..Triangle Borea BR03 comes in at just over your £300 budget. Also as others have mentioned there are great bookshelfs from Q Acoustics and Klipsch.
Others that may fit the bill (check the second hand speakers on
eBay) are the Dali Oberon, Quad S1, and others such as the Celestion Ditton 10 and 11 (great small speakers), Castle mad some great small speakers as well.
Happy hunting