Add en-GB ID Numbers (aka National Insurance Numbers)

doc/default/id_number.md

@@ -50,3 +50,25 @@ Faker::IdNumber.danish_id_number(gender: :female) #=> "050390-9980"
 # Generate a valid French Social Security number (INSEE number)
 Faker::IdNumber.french_insee_number #=> "22510589696868"
 ```
+
+## ID Number and Locales
+Besides the locale-specific ID Number methods. Faker supports retrieving localised calls to `.valid` and `.invalid`. 
+Here is an example:
+
+```ruby
+Faker::Config.locale = 'fr-FR'
+Faker::IdNumber.valid #=> "22510589696868"
+```
+
+Locales with specific intricacies are as such:
+
+### en-GB
+When provided with British English, unformatted'[National Insurance](https://www.gov.uk/national-insurance/your-national-insurance-number)' numbers are generated. 
+Note: Faker can only generate a subset of all possible legal/illegal national insurance numbers.
+
+```ruby
+Faker::Config.locale = 'en-GB'
+Faker::IdNumber.valid #=> "AJ405924A"
+Faker::IdNumber.invalid #=> "BG316764W"
+```
+

lib/locales/en-GB.yml

@@ -85,6 +85,9 @@ en-GB:
         - Northern Ireland
       default_country_code:
         - GB
+    id_number:
+      valid: '/[A-CEGHJ-NOPR-TW-Z][ACEHJLMOPRSW][0-9]{6}[ABCD]/'
+      invalid: '/(BG|GB|NK|KN|TN|NT|ZZ)[0-9]{6}[E-Z]{1}/'
     internet:
       domain_suffix:
         - co.uk

test/test_en_gb_locale.rb

@@ -69,4 +69,18 @@ def test_en_gb_postcode_outcode_is_valid
     assert_match(/^[A-PR-UWYZ][A-HK-Y0-9]/, outcode)
     assert_match(/\w{1,2}\d{1,2}|\w\d[ABCDEFGHJKPSTUW]|\w\w\d[ABEHMNPRVWXY]/, outcode)
   end
+
+  def test_en_gb_id_number_valid_is_valid
+    id_code = Faker::IdNumber.valid
+
+    assert_equal(9, id_code.length)
+    assert_match(/^[A-CEGHJ-PR-TW-Z][A-CEGHJ-NPR-TW-Z][0-9]{6}[A-DFM]$/, id_code)
+  end
+
+  def test_en_gb_id_number_invalid_is_invalid
+    id_code = Faker::IdNumber.invalid
+
+    assert_equal(9, id_code.length)
+    assert_not_match(/^[A-CEGHJ-PR-TW-Z][A-CEGHJ-NPR-TW-Z][0-9]{6}[A-DFM]$/, id_code)
+  end
 end