Did you pass underneath a gantry? Smart Motorways will be using HADECS camera's, which are positioned on the gantries. These cameras are known to sometimes go off randomly, and obviously also go off if someone is speeding in the lane next to you.
If you did set it off, depending on what end of 55 - 65 you where doing, you might not hear anything in the post. If you do, it'll be an NIP, asking you to identify who was driving at the time. If you don't fill that out, you get a seperate offence, which is worse than the speeding ticket. After sending it back, you should get an offer to do a speed awareness course, which avoids getting the points on your license. Otherwise, you can opt for a FPN, which is 100 pounds, and 3 points on your license.
The speed awareness course is the best option as a new driver, you avoid the points, no risk to your license and no insurance increase. You just spend a day in a hotel somewhere with some "driving proffesionals" who lecture you about the dangers of speeding, which depending on your attitude, can either be really intresting, or just piss you off.
Don't worry about it. You're probably fine, I went past a speedvan doing 83mph in a 70 and never heard anything. However, even if you get an NIP, don't panic, a speed awareness course is the worst that it can possibly be, and even that isn't too bad.