I have £25 in my Apple ID account from a gift card. How do I use this to buy a subscription to Apple TV ?

When you subscribe to Apple TV+, any subscription fees will be deducted from the account balance. Account balance will be used before other payment methods.

At £6.99/m, £25 should last 3 months (plus any free trial period, if you are a new subscriber). The fourth month can then be paid partially with the remainder of the balance and partially through another payment method.

How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support

Check your Apple Account balance - Apple Support

Payment methods that you can use with your Apple ID (by country) - Apple Support

Add a payment method to your Apple ID - Apple Support