The law requires automatic speed cameras to perform a "secondary check".
The GATSO lines do not have anything to do with the workings of the camera itself, it's just for this second check.
The camera takes two photos, usually 0.5 sec apart, and using this time and amount of lines travelled between the two photos (the distance between the lines being known), the speed can be manually worked out to check the reading from the camera is accurate.
Truvelo (the blue ones with three horizontal lines) work slightly differently, in that the camera pictures should show the car's front wheel on the middle line
In theory, if there are no lines, no secondary check can be performed and thus the speed camera evidence cannot be used against you. However, this would be a rather technical argument to put before a court and require knowledge/expertise/money, so unless you actually doubt the measurement, it'd be easier to take the £60 and 3 points.