For overseas ATM withdrawals:

Monzo (After 18th December): £200/month, 3% thereafter (https://monzo.com/blog/2017/10/17/atm-limits-announcement/)

Revolut: £200/month (£400 with Premium), 2% thereafter

Starling: No charge

Not sure about any of the other challenger banks, I've only used Monzo on a regular basis.

I just used a Revolut card during a two week trip around Iceland. It worked fantastically well. Iceland's interesting as some places charge in Krona, some in Euro, but in either case I was charged the live mid-market rate to the GBP with no extra spread or fees.

You're also allowed to withdraw roughly £200/month worth of cash (any supported currency) without any fees. Beyond that there is a charge, I can't remember how much offhand but it's not too crazy.

The one problem I had with the card was at their petrol stations. Most are self-service and require the card to be pre-authed before the fuel will start being pumped. Revolut doesn't support this so I had to use another card or find stations where I could pay in store.

I've got money in GBP, EUR and USD in a Revolut debit card. I've used it on ATMs in mainland Europe and continental US without problems.

you get the same thing with revolut.