In the past I've been able to amaze clients who have applied for a GB EORI number but not received anything from HMRC notifying them of it by:-
a) Having an educated guess that the number is GB<VAT number>000
b) proving it is valid using https://ec.europa.eu/taxation_customs/dds2/eos/eori_validation.jsp?Lang=en
i) Is there a standard format for the new XI EORI numbers for Northern Ireland?
ii) Is there an online checker for these that works?