I'm not that familiar with the U.K. dubstep fenomenon, but from what I've heard, I'll take a look at the X3 synth simulations (Seismik Synth, Saturn, Analog...) combined (tone 2) with the "Sub Dub" preset for starters, and see where it goes.

I had to use those simulators to cover Nine Inch Nails songs with a short lived "tribute band" and ended up with bass tones that were at the least very credible. Try experimenting a lot with those, they have a wide range from usable tight responding synth sounds to ridiculous noise makers.

I sometimes use the seismik synth simulator in addition to my regular bass tone for producing near-subsonic harmonics that emphasize and add dramatic "movie subtones" to some songs live. Shakes the earth with the right FOH subwoofers. Use with precaution...

Gonzalo