I’ve moved your post here, as it follows on nicely from the discussion earlier in this topic. I hope that’s ok 
On the face of it, this idea totally makes sense but I wouldn’t recommend it, for the reasons I’ve mentioned earlier in the thread.
The TL;DR is, it will mean that block transactions that you don’t want to block by accident & it’s just not necessary, as Monzo will refund you for fraudulent transactions, very quickly by the look of it.