Gareth, a bit of research. Please call me out anyone if this is incorrect / omissions etc
If there was an awesome free wordpress plugin - it would be rank highly on search, but nothing does.
UK postcode and address data - is licensed by the Royal Mail - and is not free**
There are resellers, who provide online lookup services and pricing / billing
Any website form you make will need to plugin to one of those services.
On some of their websites are technical guides to doing that.
**Maybe other businesses have but their own databases
I doubt they give they give away for free.
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Woocommerce - I do not see that you can use it to “just build a standalone form”. All the forms are product / checkout / account related
Gravity Forms - A popular forms maker plugin - I see no address lookup plugins on their site
Wordpress plugins repo - not much there either
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If you can setup Woocommerce for the repair booking form, there is this. I can’t vouch for it.
Or maybe use Gravity Forms and some Javascript
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is this really important to the client, like a showstopper ?
lookups pricing acceptable to them ?
are bookings made once or day or 1000 a day ?
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Phil