en-GB: Fix erroneous too strict rule for id_number (NI number)

[luke-hill]

I found this out by investigation that actually this would fail for a specific valid NI number (One that is known to be valid, but this system would detect it as invalid)

Original work / motivation from #3032

Motivation / Background

Fixed a bug whereby you cannot generate a valid NI number

Additional information

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[luke-hill]

@stefanjcollier @stefannibrasil minor item to review. Should be an easy merge and fix