Change the locality type for Great Britain to town/city.

Source: https://www.royalmail.com/personal/receiving-mail/update-your-address

const.go

@@ -64,6 +64,7 @@ const (
 	LocalityTypeDistrict
 	LocalityTypePostTown
 	LocalityTypeSuburb
+	LocalityTypeTownCity
 )
 
 var localityTypeNames = [...]string{"city", "district", "post_town", "suburb"}

formats.go

@@ -498,7 +498,7 @@ var formats = map[string]Format{
 	"GB": {
 		Layout:            "%1\n%2\n%3\n%L\n%P",
 		Required:          []Field{FieldLine1, FieldLocality, FieldPostalCode},
-		LocalityType:      LocalityTypePostTown,
+		LocalityType:      LocalityTypeTownCity,
 		PostalCodePattern: `GIR ?0AA|(?:(?:AB|AL|B|BA|BB|BD|BF|BH|BL|BN|BR|BS|BT|BX|CA|CB|CF|CH|CM|CO|CR|CT|CV|CW|DA|DD|DE|DG|DH|DL|DN|DT|DY|E|EC|EH|EN|EX|FK|FY|G|GL|GY|GU|HA|HD|HG|HP|HR|HS|HU|HX|IG|IM|IP|IV|JE|KA|KT|KW|KY|L|LA|LD|LE|LL|LN|LS|LU|M|ME|MK|ML|N|NE|NG|NN|NP|NR|NW|OL|OX|PA|PE|PH|PL|PO|PR|RG|RH|RM|S|SA|SE|SG|SK|SL|SM|SN|SO|SP|SR|SS|ST|SW|SY|TA|TD|TF|TN|TQ|TR|TS|TW|UB|W|WA|WC|WD|WF|WN|WR|WS|WV|YO|ZE)(?:\d[\dA-Z]? ?\d[ABD-HJLN-UW-Z]{2}))|BFPO ?\d{1,4}`,
 	},
 	"GE": {