And when you are requesting a certificate for foobar.bank.com, your certificate request must come from an authorized email address at bank.com. That is also where your issued certificate would be sent. So, in order to get a certificate from a third party issuer, you have to:

• Control the domain registration at the level just above the TLD (I don’t know how it works for co.uk, probably similar though)
• Have access to a mailbox at the domain, where that mailbox has an address which is authorized to request certificates (this would be configured in the domain registration)

Could a malicious actor compromise that mailbox in a way that allows them to request a certificate and then receive it? It’s not impossible, but it would be a huge effort with a small payout. Honestly, if you’ve got access to that mailbox, you don’t want to give yourself away by making false certreqs through it. You want to just exfiltrate as much data from it as you can. There’s certainly something way more valuable in there.