Pre-approvals on Monzo are a continuing frustration of mine, not because I can’t use the funds, but because you don’t know what is pending and what isn’t without digging into each transaction. Now that petrol stations are pre-authorising the full amount rather than a quid, I am fairly sure they won’t finalise the correct amount until days later, making my Monzo account look overdrawn when it isn’t, regardless as to what Mastercard and Visa say (good luck with that 20 minute turnaround 
not gonna happen)
For more thoughts on this topic, take a look at Pre-ordering authorising the full amount months in advance which is about pre-ordering with CDJapan (who place a hold on funds for 30 days at a time, but don’t actually finalise until the item ships) which is different to pay at pump, but the way Monzo surfaces things like this are the same thing.