The ISO certifying authority is called the CB, which operates under the auspices of the AB and is accredited. Finally, this AB operates under the supervision of the IAF Global Creditors Organization and is also accredited and managed. Given this, it is certainly clear that IAF-certified certifications have the highest level of validity.
AB of England is the UKAS organization and to access the list of ABs approved by the IAF world organization, you can refer to this organization's website which is in the related links section in the footer.