Janspeed do a manifold for the K11 which will set you back around £200, they do a de-cat at £80 and a cat-back system at £200. This would probably be the best option as you gain power and get a nice sound with the only downside being its mild steel. You will gain somewhere near 20bhp if you have the 1.3 engine by doing this.
Maniflow also make a manifold for the Micra but this is much more expensive.
Seeing as you really just want "the right sound" the best bet is to possibly just get a backbox as it would be cheapest. If you can strech it go for a stainless cat-back system, you practically state the system you want, when you want it loud (low/high revs, motorway/town driving etc) and it will get made for you. This will provide a slight performance increase and will come with a liftetime guarentee possibly, expect to pay in the region of £300 for this.
Powerflow is the one I can think of as they have dealers dotted around the country but have a look in Yellow pages or yell.com as it might have others around your area.