Also bear in mind, that there are very very strict rules on how much bags can weigh due to H&S lifting rules. So i think you will find that all responsible Checkin staff WILL weighbags to ensure they are within the limts and if they need weight tags. They will therefore also be able to see if you are over your limit, and with the economic squeeze on for airlines, every £1 helps so no reason for them not to charge.
The only possibility is to stand in the longest queue full of impatient looking people, and hope the agent decides its not worth the tutting and moaning from the queue. But you may just get charged having stood in a heinous queue and then getting abuse from the people behind you!
Best off just sticking to the 15kg!