Administratively, assuming the goods were either manufactured in Spain and/or had duty paid on them (in free circulation), you could have them shipped over to you with little to no administrative fuss. If we leave the Customs Union, Spain becomes a third country and you'll need to make a customs declaration or have an agent do one for you. All the interoperability and shared best practice that has come about over the decades will disappear.

Even something as (not at all) simple as valuation is affected now - Customs Duty is calculated on the Cost, Insurance, Freight (CIF) value of goods and at the moment the freight costs you need to include are calculated with reference to the EU border. Now there won't be an EU border, it'll be the UK border and that could add significant cost or most likely, avoidance.

It also comes at a really bad time for the UK Customs authority who have been planning to replace their Customs system for years and were in the process of doing so.

It's a nightmare and it's frustrating to watch politicians talk about it when they clearly have no understanding of the mechanics, some don't even have a basic grasp of the fundamentals like what is actually an import/export for Customs purposes.