The Fast Lane security channel has been running at Liverpool since July 7th last year. It operates no differently than fast track at say Manchester Airport where you do also go through the same equipment and cut to the front of the queue. Only difference is at Manchester and other airports you have to buy it as part of a so-called Business Class ticket whereas at Liverpool and now Bristol you can buy it separately.
The process is just like eurodisney and following Liverpool's success with it it was only a matter of time before others offered this service. Works because the standard queue is no worse in queue time than it was prior to fast lane. Nice to have choice. But maybe £5 is too expensive to get significant take-up.