A $150 hold would clearly be noticed by the victim, so the attacker wouldn't even try it.

Yeah… no it wouldn’t. I’ve watched users have their bank accounts emptied (by accident) because they kept refreshing. A measly £150 isn’t going to register until it’s too late anyway.

Maybe if my bank emailed me, otherwise I doubt it. Local gas stations routinely use $200 holds and I'd have to go way out of my way to see it happen.