Really there are quite a few options open to you here:
Either
1) Keep the Magnex back box and simply buy a 'Straight Through' centre section (Without Centre siliener) hence making the exhaust louder that way.
These can be found on places like eBay and a number of manufactures do these or you can get a custom exhaust shop to fabricate you a straight through pipe.
2) Go for a full off the shelf stainless steel performance exhaust, manufactures include Pipe, Scorpion, Janspeed, Magnex, OMP, Mongoose and a few companies that produce a inhouse item such as Cituning.
Most of the above will be from the cat back with a centre box and back box, most will offer a range of tailpipes which is more for personal choice rather than performance. Again most of these leading brands will set you back between £230 upto £300+ depending on tailpipe choice and brand, also shop around as some brands can be found cheap else where!
Quiet systems are: Magnex and Janspeed
More throaty systems are: Piper, Scropion and Mongoose
Loud systems are: OMP which is a more competion item and comes in at about £300 and has a 2" tailpipe.
3) option (which some dissagree with) is a custom build either a full tailpipes exiting where you want them or sourching your own back box either universal or from another car and have them fabricate all new pipework, these are seen as more looks than performance really.
In my personal opium as members to post pics of there exhausts 'Except Magnex' as you already have one, ask for Piper, Janspeed, Mongoose and anyother stainless steel exhaust people mite have and then see what you like and what you dont :-)