The Onkyo HTX-22HD is a high definition, 2.1 channel (upgradable to 5.1), home cinema speaker system featuring HDMI 1.3 connectivity, Dolby TrueHD, Dolby Digital Plus and DTS-HD Master Audio decoding, an integrated amplifier/subwoofer and stereo front speakers.
Depends what you are looking for. For £300 there are many all in one box solutions although the Onkyo one mentioned above is probably one of the best.
£300 would be very tight for a 5.1 seperate system although if you went the secondhand route then it may be possible. Could also go AV reciever 2 or 2.1 to start with then add more speakers as funds allow. Av recievers (5.1 Dolby Digital/DTS) can be had new from Richer sounds for £200. You can then buy speakers for what is left (my first set of speakers cost me around £100 for 5 speakers- not the best quality but got me started). The subs can be had for around £80, again not best quality but will get you started.