birthday flashmob

Hi there, I am coming to London from the states to visit my boyfriend for his birthday and would love to surprise him with a flashmob. It would be indoors and I already have the ok from the restaurant. I am looking for around 15 or 20 people to be involved. Right now, I am pricing this out and would like more information on cost. I don't have a lot of money and have thus far called in favors from friends but would like to know whats possible from you guys on a limited budget. thanks so much, Susan