Talk about being late to the game... iZettle has been around since 2011[a]!
Interestingly Square seems more expensive than iZettle. Square is flat 1.75% in-person 2.5% online while iZettle is sliding 2.75%-1%. The threshold seems to be £6200/month at which point iZettle is going below 1.75%. That's pretty low so I suspect most shops would still chose them.
Guessing they've had offers, but not good enough to make them sell and they're still able to raise capital.
Square's market cap is currently $6.51B and here's there balance sheet: https://markets.ft.com/data/equities/tearsheet/financials?s=...