LGW fast track ("Gatwick Premium") is now much more variable due to the sheer volume of people using LGW now (LGW keeps having record passenger numbers passing through) and the various ways you can obtain it - e.g. if you pay for lounge access at the No.1 Traveller lounge, it includes Premium lane access, or all the BA/Oneworld elites.

If there's a queue, it's better to go through normal security as LGW have vastly improved the normal security channels with lots of additional channels, whereas Premium only generally has one.

The situation should improve once BA move entirely to South terminal next year.