Well, if deep and loud are what youre after, and you dont mind mild steel, then Ashleigh would be amazing for about half your budget. Its too loud for some peoples tastes though.
Yo could get a Supersprint for probably about that money, that should sound pretty good.
If its just a backbox youre looking for, and you want a lot of noise, I wouldnt rule out Peco. If you go for the stainless Boxer one and choose a subtle tailpipe (ie. oval or twin, rather than the 4" Big Bore one!), that should give you a nice sound, possible power gains and a lifetime guarantee and it shouldnt cost more than £150.