Install this plugin for UK address validation on Opencart registration page and checkout pages from 3x Software & Royal Mail.
Sign up for a trial account that includes 30 free credits at www.postcodes4u.co.uk.
Enter the username and the product key generated from the www.postcodes4u.co.uk site into the plugins configuration screen and have address validated data for your customers orders.
After the trial period (30 credits) has expired, it's easy to purchase more credits from a range of competitively priced bundles to ensure your dispatches are going to a UK address that has been PAF validated.
See this plugin in action on our demo opencart site, just select United Kingdom from the country drop-down
(Tested in v 18.104.22.168)