I'd recommend approaching bars or pubs where you drink at least semi-regularly, or preferably know one of the staff. If you know the staff and clientele it's much easier to explain what you would play and why it will be great (or at least inoffensive, which is what a lot of bar managers are looking for, ultimately).
As Mister Sloane says, choosing less busy slots is the best route in - most venues will have Friday and Saturday night slots filled already. It's often a good tactic to ask what their quietest night is in terms of takings, and go for that. If you can get 10 or so mates (or Dissensians) to come along and buy 2/3 drinks each, that's an extra £100 quid+ in takings, of which you can reasonably claim £20 quid and/or drinks. If the manager and staff like what you do, you may get offered more prestigious slots and therefore more money.
No thoughts as to specific venues, but if you see that a place has just opened or changed hands then they'll often be looking for new DJs. Oh, and be prepared to play off CDs or your laptop - fewer and fewer venues have turntables these days, and those that do often don't maintain them very well.
