Slimelight is London's longest-running Goth club (though some would dispute just how much Goth music gets played there these days). It's the social centre of the London net.goth and wider scene, has been running for (near to twenty) years and is known worldwide.
There is a members only policy. To get in you have to be a member, or a guest (i.e. a member must sign you in), although sometimes the latter requirement does not seem to be enforced . To become a member you must be sponsored and seconded by two existing members. Rumours of a test are apparently unfounded...
Entry costs �5/�8 for members/guests, and membership costs �10 annually.
If you're not a serious goth or don't know anybody, you may not like it; regulars don't want to hear the same overplayed tracks again and again, so you may find the music tends towards the obscure at times, and many people there already know each other, so it can seem unfriendly to newcomers.
If you wanted something less Goth, or less cliquey, you might like Full Tilt.