Only need to wake the phone for Google Pay, don’t have to unlock it generally. Though higher amounts supposedly you have to unlock it but I find the terminal still limits to £30 when I’ve tried.
Barclays cards don’t work with it as they have a disagreement with Google. Barclays have their own app, though apparently this has higher limits.
Also on the tube, there’s more of a delay tapping in, it’s faff to wake the phone first and most times it would fail on first tap so have to do it again, much to annoyance of those behind. So I use a card instead. Remember to tap out also and use the same card or phone you tapped in with!
The original defined standard NFC payment system worked even if the phone was off or out of battery as it relied on a special SIM and antenna in the battery, but then there’s no app involved that Google / Apple can harvest usage data, so it wasn’t popular.