do i connect with coexistence? does it work in UK?

doesnt work in UK, will try another method

what does this option mean?

Hello ​@Mikeymike ,
This option should be selected when you already have a phone number and want to use it with manychat.
By choosing this method, your number will not be accessible through the WhatsApp Business app, and you will not have access to any previous message history if the number was used before. It will be used exclusively within manychat, both on the desktop version and in mobile apps.

The manychat help article below explains each of the three available methods, helps you choose the right one, and provides step-by-step instructions on how to connect using your chosen method:
https://help.manychat.com/hc/en-us/articles/19247843644060-How-to-connect-WhatsApp-to-Manychat-choosing-the-best-method