Look for a normal UK provider that supports "wifi-calling".
For US numbers, use Tello's basic e-sim: it supports "wifi-calling" i.e. as long as you're connected to the internet, you receive & send calls & texts as if you're in the US, no matter which country you're actually in. Because it's just a normal number, it'll work with WhatsApp.
$7/month for unlimited text, 100 phone minutes, no data.
For US numbers, use Tello's basic e-sim: it supports "wifi-calling" i.e. as long as you're connected to the internet, you receive & send calls & texts as if you're in the US, no matter which country you're actually in. Because it's just a normal number, it'll work with WhatsApp.
$7/month for unlimited text, 100 phone minutes, no data.