Each country will have a Data Protection Authority (DPA) which is the regulator in the country. The ICO is the one in the UK.

The last letter of the GDPR is Regulation. A regulation is very different than than a Directive (the pre-GDPR law is based on a directive). There is very little wiggle-room with a Regulation, even between countries. The ICO also works with other DPAs currently as part of Working Party 29, which ensures the DPAs are working in Sync.

So the ICO advice is worthy of close study, especially if your local DPA (assuming you have one) has not commented or given guidance on a certain matter.