The Milltek with HFC/decat pipes is very popular on here, kind of OEM sounding until you open her up then it's bliss (apparently, i've not heard it)
Cobra (resonated) is a very popular choice, good price (£425) for Y pipe back but the Y pipes have went up by about 40% to £275 I think. Similar to the Milltek only a little louder. Very good quality, british made and choice of tips.
I myself have the Invidia Gemini which is true dual with a cross over pipe in the centre section. Choice of tips again but these include titanium style ones. Not heard it side by side with the Cobra but it has a nice note low down and roars when on full chat Price is pretty good at about £850 from our traders.
I wouldn't touch Crapspeed with a bargepole after the negative reviews on here, too many fitting and quality issues.
Scorpion is pretty similar to the Cobra I think, has a nicer large back box but only comes with slash cut tips, so depends if thats your thing or not. Price is a little cheaper than the Cobra at about £375.
Thats all I can think of right about now, except for the Ark system which looks the dogs, and is similar to the Invidia I guess, but not heard it yet. Think the price is about £800.
There are of course other options, but these are the most popular.