WooCommerce.
UK address lookup on the block and classic checkout, plus My Account addresses. No configuration.
WooCommerce ships its own address fields, so the plugin finds them and wires them up the moment you save your API key. That covers the Cart and Checkout blocks, the classic shortcode checkout, and the address book under My Account.
Three steps.
Install the Postio plugin and activate it.
Go to Settings → Postio and paste your API key, then press Save key.
That is the whole setup. Open your checkout and start typing a UK address into address line 1.
Details.
Both the billing and shipping forms are wired independently, so each gets its own suggestions.
Non-UK shipping countries are left alone — the finder only offers UK addresses.
Using something else?
Every Address field on every form, wired automatically.
Automatic on WPForms Pro. On WPForms Lite, two minutes with CSS classes.
Native Address field, free core, wired automatically.
Native Address field with sub-inputs, free, wired automatically.
CF7 has no address field, so you tag yours. Two minutes, once per form.
No address field in Ninja Forms, so you tag yours. Two minutes, once per form.
Elementor Pro only. Tag your fields with CSS classes and you are done.
Ready in under a minute.
Sign up, grab a key, paste it in. Your first hundred lookups are on us.