With regard to doing a spot in a club ( wether London or any where else) simply tell who ever is on the door that you are prepared to do
a spot and they will either take your name or refer you to who ever is
running the evening.Its as simple as that. And of course DONT disappear into the bar and ignore the rest of the singers when the evening gets started . You never know when the 'programme' might need an alteration halfway through the evevning.