The main UK Flight Connections is a separate area for dealing with passports and getting your biometric photograph into the system. After that you rejoin the rest of Flight Connections to go up to North Security.

You can only use South Security if you go through the UK Border next door as an arriving passenger, then go to main South departures from landside. This would be sensible for a few reasons: if Flight Connections looks very busy around the up escalator, or if you smoke, and if you have an EEA or Swiss passport, thereby allowing you to use the e-gates.

If staying airside, which is the advertised route, then Fast Track security is pointless at North Flight Connections, it does exist but the queues there are invariably the longest in the airport. So when you get upstairs into the security hall, just keep walking as far as you can, the queues get shorter the more northwards you go. If you have a single booking the domestic leg may be booked into the Business UK fare group and that would entitle you to fast track security, but as I say it's not worth worrying about in this instance.

The North Club lounge is right by that security point, just keep going northwards once through security.