Fast Track is indeed a joke.

Fast Track Flight connections is not available for domestic transfers, only international. This is because there is no immigration check in that lane. So if one wishes to enter the UK without a valid visa...well I'm not going to spell out how to do it publicly but it is very straightforward.

Then as Johan Rebel says if one uses the 'Fast Track' escalator one just joins the melee at the top anyway. And I doubt very much whether Fast Track is in fact any faster than regular.

It's like everything else to do with security at T5, smoke and mirrors.