Quite a space of time since any posting on a Postcode Address File (PAF) lookup integration.
Previously @Glenn has posted of the cost implications but there are other official address lookup options available these days.
Ordnance Survey have introduced API products; such as OS Places API | Data Products | Ordnance Survey that maybe a more economical and easier to sync than the 4 CD updates used historically. A new feature that I recently discovered within addressing is the Unique Property Reference Number (UPRN) if a UPRN data-field was an option for client and supplier records there would be an easier method to link the client and supplier as they at the same address…
An additional benefit of connecting to such an address lookup system is that the local authority is included which would fix Welsh border postcodes for the correct flag to show when looking at client/supplier records (Postcode areas CH5-CH8 change of flag)