diff --git a/src/server/plugins/engine/components/helpers/helpers.test.ts b/src/server/plugins/engine/components/helpers/helpers.test.ts
index e597fa0d1..9ed971030 100644
--- a/src/server/plugins/engine/components/helpers/helpers.test.ts
+++ b/src/server/plugins/engine/components/helpers/helpers.test.ts
@@ -9,7 +9,8 @@ import { OsGridRefField } from '~/src/server/plugins/engine/components/OsGridRef
 import { createComponent } from '~/src/server/plugins/engine/components/helpers/components.js'
 import {
   createLowerFirstExpression,
-  lowerFirstExpressionOptions
+  lowerFirstExpressionOptions,
+  lowerFirstPreserveProperNouns
 } from '~/src/server/plugins/engine/components/helpers/index.js'
 import { FormModel } from '~/src/server/plugins/engine/models/FormModel.js'
 import definition from '~/test/form/definitions/basic.js'
@@ -145,6 +146,42 @@ describe('ComponentBase tests', () => {
   })
 })
 
+describe('lowerFirstPreserveProperNouns', () => {
+  test('should preserve "National Grid" capitalisation', () => {
+    expect(lowerFirstPreserveProperNouns('National Grid field number')).toBe(
+      'National Grid field number'
+    )
+  })
+
+  test('should preserve "Ordnance Survey" capitalisation', () => {
+    expect(
+      lowerFirstPreserveProperNouns('Ordnance Survey (OS) grid reference')
+    ).toBe('Ordnance Survey (OS) grid reference')
+  })
+
+  test('should preserve "OS" capitalisation', () => {
+    expect(lowerFirstPreserveProperNouns('OS grid reference')).toBe(
+      'OS grid reference'
+    )
+  })
+
+  test('should lowercase first character for regular text', () => {
+    expect(lowerFirstPreserveProperNouns('Enter your name')).toBe(
+      'enter your name'
+    )
+  })
+
+  test('should handle text without special terms', () => {
+    expect(lowerFirstPreserveProperNouns('Latitude and longitude')).toBe(
+      'latitude and longitude'
+    )
+  })
+
+  test('should handle empty string', () => {
+    expect(lowerFirstPreserveProperNouns('')).toBe('')
+  })
+})
+
 describe('lowerFirst expression helpers', () => {
   test('lowerFirstExpressionOptions should have lowerFirst function', () => {
     expect(lowerFirstExpressionOptions).toHaveProperty('functions')
diff --git a/src/server/plugins/engine/components/helpers/index.ts b/src/server/plugins/engine/components/helpers/index.ts
index d7dd1a748..88e4c78de 100644
--- a/src/server/plugins/engine/components/helpers/index.ts
+++ b/src/server/plugins/engine/components/helpers/index.ts
@@ -39,12 +39,24 @@ export const addClassOptionIfNone = (
   options.classes ??= className
 }
 
+/**
+ * Applies lowerFirst but preserves capitalisation of proper nouns
+ * like "National Grid", "Ordnance Survey" and "OS".
+ */
+export function lowerFirstPreserveProperNouns(text: string): string {
+  const result = lowerFirst(text)
+  return result
+    .replace(/\bnational [Gg]rid\b/g, 'National Grid')
+    .replace(/\bordnance [Ss]urvey\b/g, 'Ordnance Survey')
+    .replace(/\b[oO][sS]\b/g, 'OS')
+}
+
 /**
  * Configuration for Joi expressions that use lowerFirst function
  */
 export const lowerFirstExpressionOptions = {
   functions: {
-    lowerFirst
+    lowerFirst: lowerFirstPreserveProperNouns
   }
 } as ReferenceOptions
 
diff --git a/src/server/plugins/engine/pageControllers/validationOptions.ts b/src/server/plugins/engine/pageControllers/validationOptions.ts
index 2c20a6b3e..da35f0489 100644
--- a/src/server/plugins/engine/pageControllers/validationOptions.ts
+++ b/src/server/plugins/engine/pageControllers/validationOptions.ts
@@ -8,11 +8,12 @@ import joi, {
   type ReferenceOptions,
   type ValidationOptions
 } from 'joi'
-import lowerFirst from 'lodash/lowerFirst.js'
+
+import { lowerFirstPreserveProperNouns } from '~/src/server/plugins/engine/components/helpers/index.js'
 
 const opts = {
   functions: {
-    lowerFirst
+    lowerFirst: lowerFirstPreserveProperNouns
   }
 } as ReferenceOptions