I've just submitted v3.1.0 of VAT4EU for the plugin moderators' review; I'll post back here when it's available for download.

This release contains changes associated with the following GitHub issues:

#13: Various re-factorings; the minimum PHP version is now PHP 5.6.0.
#14: Use zen_is_logged_in() and zen_in_guest_checkout() to determine customer's logged-in status; VAT processing now only for non-guest customers.
#15: Since Brexit, GB is no longer part of the EU; the VIES service no longer recognizes GB-based VAT numbers.