Is this PC Worth £300 ?

I would say its worth it, if it was from a reputable source. Because you can always upgrade it. The main thing holding it back would be gpu, but thats easily upgraded. That processor seems very weak though, and is likely an old socket.
 
Maybe in the sum of the reusable parts but as an operating PC, I'd say no. The "quad core B55" CPU is actually a Athlon II x3 455 which has had the 4th core unlocked - it's about 3 generations old now. The graphics is decent but there is no telling about the PSU nor do I see any guarantee or warranty of any kind... I'm just not liking it personally