BT is only a virtual mobile net operator, so will not appear in the basic network search. That will reveal only true NetOp's and not virtuals or SimOps (SIM Operator - Tesco, GiffGaff, etc)
BT leech onto EE as the NetOp. If you are in the UK, and use a SimOp like BT who only use a single NetOp like EE, then it doesnt matter if you have manual or automatic selection set - Only EE is available.
However if you use a SimOp like Anywhere or Manx, then it is relevant, as your SimOp can use any available NetOp
