Do a manual channel search for 12519. Prime, Bravo+1 etc are on one of the Sky Transponders.
However if you did an Auto search for FTA it should have been found during initial setup.
P.S. Your profile is a bit confusing. You say you receive Freeview HD via UHF yet you are using a Satellite STB (Supernet 4200S) which won't work on UHF. It needs a satellite dish and Satellite Freeview is only SD not HD.
You are right. We have both but must only be connected to a satellite dish. So what frequency do we programme for Prime?
As stated in the previous reply. 12519