At the minute, you can't get harder drum and bass than Current Value and Donny. Any darker and harder and you're into the realm of breakcore, the likes of Venetian Snares.

That said, angeruk.net is the management for some of the UKs hardest producers and DJs. My personal favourites from Angeruk / freak recordings and others are as follows:

Audio, B-Key, Current Value, Donny, Dylan, Gein, Limewax, Robyn Chaos, SPL, Spor, Technical Itch, The Sect, Unknown Error,   there's more. But I can't be arsed to list them all, but those are my favourites.

Then there's the Therapy Sessions which pretty much every DJ / Producer on the Angeruk roster has a part in.

Also search for those artists and therapy sessions on youtube. Current Value has a lot of tunes people have upped onto youtube that aren't on his myspace.

If you want to go harder. Breakcore. Venetian Snares, End.User (both personal favourites). Or speedcore.

And then you go WAAAAAY off the scale after breakcore, with hardcore experimental which is just pure fucked up. Not even I can listen to the shit and I like some fucked up music. An example for reference would be Hybrid Frequency Noise listen to the song "Musical Shadow". When I say song, I mean.... well noise.