They wouldn't be able to detect you from the street. The BBC would know which IP addresses are logged on to iPlayer, but they would have to contact your ISP to get your address then cross-match it against the TV licence database. Least I think this is what they would have to do.
I don't think there is anyway they are going to able to do this. Not worth the effort.
This might be all irrelevant anyway as half of the population of England will be trying to get onto iPlayer at 3pm, be very surprised if it manages to cope with the demand.