To be really unlocatable, one needs not just to switch off a gadget, but also remove the battery
Not on an Apple you don't. Nor indeed almost all new Androids.
GPS-proof covers are a start.
But remember, the more "unsual" things you do, the more you stand out of the herd crowd. Something my slightly tinfoil hat DBs have never grasped. Out of all my contacts, they are the only two that insist on PGPing all their emails.
If you were sniffing for unusual behaviour, where would you start ?
Really savvy bad guys would start with an a priori assumption they are being watched anyway. So you'd never catch them by observation. (The ongoing insistence of the security services of hoovering up electronic data is commendable, but completely useless against a terror cell who are communicating with adverts in code in Farsi or Urdu, or Gujarati placed in newsagents windows in busy city centres).
Anyway, as we've seen. Even when the security services do get it right, and identify the right bad guys, the UK hardly becomes a safer place.