Once the voucher is issued, it ceases to be connected to any terms and conditions of British Airways American Express eligibility.
So, although a non-UK based Executive Club member cannot apply for or continue to use a UK-issued British Airways American Express card, any BA Miles or vouchers previously-earned remain valid and available for use within the member’s account.
Accordingly, moving to Europe during the validity of a 2-4-1 voucher will not affect it in any way.
Likewise, a voucher which has been redeemed will be re-issued to the holder upon cancellation of the associated booking, subject to the original expiry date not having been passed.
I’m not aware that these protocols are written down in the Ts&Cs as such; they simply result from system practicalities and associated precedents.