try listening to the evening programs on radio 1, steve lamacq and huw stephens shows seem to be open minded. and also i think zane lowe and jo whiley like to hear from bands. not sure if they'll play them, but they like to get cds.
Clint Boon on Xfm Manchester is always after demos and regularly plays unsigned bands. The station runs a weekly vote on some of the better demos with the winners going on to act as support at a 'proper' gig.