The Bose system is chod, full blown cack out of the bum. If you replace the speakers and not the sub then you will be immensely disappointed as it will not do the system justice. It sounds decent at present because you have the standard speakers distorting up high and the bass is.....well.....bass I guess! You can pick up some subs that will go in without needing to do anything other than replace the original and wire it up, free air I think they are called. Trust me, you'll be better off doing it.
For £400 I would just disconnect the rear speakers and go for a set of components with a sub. Coaxial a for the rears are honestly not necessary and if you really must have them then you can always add to the system at a later date.
Anyways you're in luck when it comes to top quality audio installers in Leicester as you have a Four Masters registered dealer nearby. Plush Automotive are unbelievable and as it happens one of my mate's cars represents them at quite a few shows throughout the year.
Their address is:
126 Station Road, Broughton Astley, Leicester, Leicestershire, LE9 6PW
Not only will they do a proper job, they will set the system up to however you want it to sound. Look out for photos of Paul Bailey's Golf R32 in the shop, he's spent a lot of money there!
For your money you should be looking at Hertz, Morel or Rainbow components, Hertz or Pioneer subwoofer and maybe an Alpine amp. I have some more detailed info that I will PM you with.