The UK Government (after OSM took off) started to open source and release more data. You can download[1] the database of most of the UK postcodes[2] and their latitude & longitude. So if you want to really verify a GB postcode, you can just check it against the list

[1] It's called 'Code Point Open' and you can 'order' it (for free) here http://www.ordnancesurvey.co.uk/business-and-government/prod...

[2] It's most of UK. It doesn't include Northern Ireland