If you're on the covers circuit, a lot of the chain pubs and larger places will only book through agents, and its strictly covers. Obviously for pubs booking entertainment themselves its a different ballgame, so ask them directly.
With regards to original music, most venues will either have inhouse promotions, or will have a number of promoters in the area that put events on there. So take the Barfly in Camden for example, as far as i know, it (or any barflys) doesn't have inhouse promotions, they deal with external promoters. One example who put nights on at the Camden Barfly is 'KBY Rocks', affiliated with Rocksound magazine. I've played a loada gigs with them, and they're great. Note that this is Original Music only!!
Just look up promotion companies on Myspace!!!
Si
p.s.
oh and usually, i would suggest that all promoters of live entertainment will want to hear a recording of you, unless perhaps if you get recommended by someone else, but even then it wouldn't surprise me if they asked for a recording