Apologies for the late reply. Check if there are any places available on the next intro ride. If there aren't and you're confident you can ride in a group safely, feel free to come along on a club ride. We have a guide to group riding which lists all the relevant hand signals which you may find useful - http://www.islington.cc/#faq
Of course, the intro rides are really the best place to learn these and you can always come on a club ride then attend the next available intro ride.
It sounds like you're in good shape so it's worth noting that if you attend one of the faster rides, signalling and communication is sometimes not as thorough as a higher level of bike handling is implied by the pace. Get there early and let the ride leader know that you're new. If you have any questions about signalling, the leader or any other member should be able to help.