Then if you want a sub. Drop those components if you're still sticking to £300. Also drop the amp. Now subwoofers are tricky... Get very expensive for quality see....Then you have to consider boxes for them...I have a sealed box. Which i custom made, in it i had a cheap subwoofer and i loved it with my standard speakers and my aftermarket headunit. However, now i have my new components they go down to 60hz. And when they are amped i don't really require my subwoofer. So i took it out.
Subwoffers if you buy them active they tend to provide no where near the power they state. So i'd be inclined to get a passive one and use a separate amp. My mate has this, he thoroughly recommends it, however its going ot be raping on your budget of £300 if you want sound quality.
http://www.caraudiodirect.co.uk/vibe-cbr15-p-2446.html
When you need an amp to power it -
http://www.caraudiodirect.co.uk/alpine-mrpf300-p-6474.html
That will power both the sub and components, however, it will only leave you £60 for components and sound deadening.
If i were you i'd just get a good set of components. Many to choose from on CAD and then i'd amp them up and sound deaden the doors. Sorted.
It's clearly your choice, but you may also want to read this thread....
http://www.cliosport.net/forum/showthread.php?t=328154
A discussion last week of sub or no sub.