I know this is an old thread, but in case anyone is doing the rounds like I am, figured I would contribute.
If you are going to get an exhaust for noise, a simple backbox replacement or a catback exhaust will suffice.
If you want power gains, then it would be good to upgrade the header too so everything flows out of the engine smoothly and as fast as possible.
If you get a custom exhaust, make sure you try to keep the pipe as straight as possible and if you can, get it mandrel bent, not crush bent to avoid unnecessary flow restrictions.
A lot of folks say the first engine mods to do are I/H/E. Intake/Header/Exhaust. If you are sucking in more air you are going to need to expel it more quickly to make performance gains.
Oh, to stay on topic.... doing those mods probably won't fail you on MOT. Although I wouldn't recommend getting your MOT soon after getting a new exhaust as they can smoke a little when they are new until they carbon up a bit.