Not sure if things have changed since then but as of late February, Fast Track was closed at T5 flight connections, as were the escalators up to South Security - everyone was routed through the standard lines and up to North Security (where the lines were very long as they were testing new screening technologies that seem to have a very high false positive rate). That said, Fast Track was open at South Security if you are originating in London or have the option of going landside.