Why don't you tweet the codes or post them on your facebook profile? That would seem a fair system where all your followers can get the code at the exact same moment!
Posting them here also gives everyone equal opportunity to book tickets. The newsletter takes a while to get to everyone and because of that seems the least fair option. Am I wrong? Am I wrong?
Today my newsletter arrived 12.20, too late to get tickets in London. And by the way, why don't you use different locations sometimes? Shaftesbury Avenue is crap.