Starling might be the same as Monzo, no overdraft, payment refused.
I don't know about Monzo, but I set up my Starling account with the app very quickly and easily and I'm a bit of a luddite technology wise.
The video thing where you had to record a short video of yourself as part of the identification process slightly freaked me out, but appart from that it was fine. You earn a bit of interest with Starling and can spend or withdraw money worldwide without any commission or charges, so I'm going to use it for that mainly for overseas spending and move away from my trusty Halifax clarity card, which I've had for years, but Starling will be slightly more convenient as some providers charge for credit but not debit cards, and I can just dump my holiday spending money in the account instead of having to transfer over to the Halifax account afterwards, to reduce the interest charged. Also better as not withdrawing cash on a credit card, which is seen as a bad thing, even if you pay it back straight away.
Overseas spending wasn't what you asked, but I really like Starling, don't know much about Monzo, but it seems that they both offer something very similar and easy/convenient/useful and the sort of thing that you are looking for.
The Starling card is a very fetching shade of pale greeny/blue though .